I've been thinking of doing a post like this for awhile, and this is as good a time to do it as any.
First a bit of history. I first started playing D&D 25 years ago starting with the classic Red Box Basic Set. I was the DM and have spent most of my playing time as the DM.
I then 'graduated' to AD&D 2e when it first came out, and only very briefly played 1e a couple of times just before that. We played that for about 3-4 years before getting seduced by Torg, and then played in two AD&D campaigns while I was going to school in Montreal. I only briefly played 3e in Seattle when I was living there but started DMing 3e when I moved back to Kamloops in 2003. By the time 4e was just about coming out I was souring significantly on 3e. I wasn't happy with how easy it was for one class to dominate the party. I didn't like how long the combats were taking at higher levels. I didn't like how the monster blocks were massive, largely because of having the monsters follow the same generation rules as PCs. They also often didn't get to do much before they died. I didn't like how the epic boss fights were often anti-climactic.
So I was very enthusiastic about 4e as the spoilers starting coming out and bought the three core books on release day.
Over the past 21 months I've now got a good feel for what I like and what I don't like about 4e.
What I Like
General and PCs
1. The 'math' works. It is very transparent about what sort of level does what sort of damage, or to hit modifier. From what I can see the 'sweet spot' really does encompass all levels, whereas in 3e it was pretty much level 5-12 or thereabouts.
2. In-Combat and Out-of-Combat abilities are separated. Specifically for powers vs. rituals. No longer do you need to worry about memorizing Fireball over Shadow Steed (and we all know what wins out there). You can choose both.
3. No crunch bloat because of D&D Insider. Being able to keep all the powers, items, monsters in a database makes the multitude of splat books actually usable. In 3e I hardly used anything beyond the Core books. In 4e I allow almost everything.
4. Roles. While a lot of people don't like them, I do. As long as you cover the roles it doesn't really matter what class fills them. Yet the classes do have different flavours. No one is forced to take a Cleric to provide healing. No one is forced to take a Rogue to deal with traps.
5. Electronic tools. The Character Builder can be fun just by itself. Same with the Monster Builder. Join those up with some combat utilities run on netbooks and running an encounter can be a breeze. The corollary though is that without the Character Builder I'm not sure we would have stayed with 4e to begin with. It really is that necessary.
6. Clerics would usually use their spell slots for heal spells. I found they rarely used anything else. Yet they have a variety of powers now, and they can do damage at the same time as healing.
7. You don't have to rely on Clerics for your healing. You are capabable of doing it yourself (although the classes of the Leader role are much better at it).
8. I like having the skill list narrowed down. It wasn't really necessary to have a separate skill for 'use rope'. 3e did at least bring skills into the core rules as opposed to the non-weapon proficiencies which seemed like such an addon to 2e.
9. Despite having a multitude of spells in 2e and 3e, you still had just a handful of 'obvious' spells. Enlarge. Haste. Fireball. Lightning Bolt. Invisibility. Silence 15' Radius. Continual Light. Teleport.
10. Psionics is an integral part of the core rules now. It's not like an add on that it was in every other edition.
11. Artifacts are just awesome. They have lots of flavour (most of them anyways).
12. Cosmology. I really like the new way the planes are set up. They're actually useable.
Monsters and Encounters
1. Changing monsters, even on the fly, is a snap. This allows you to very easily alter published adventures for your players current level. You also don't need to 'wait' for a level to run a certain monster. Want to run a Level 15 Beholder? No problem.
2. Making monsters is fun. I've never made new monsters before. I've already made several in 4e. Part of this is because the math for balancing the monsters is transparent.
3. A Kobold is not a Kobold. No longer are all the humanoid monsters just reskinned humans. There can be a variety of kobolds, zombies, goblins, etc.
4. Boss Encounters. No other version of D&D does the big epic battles as well as 4e. It's not even close. Even if Elites and Solos aren't offensive enough as written, they can at least survive long enough without being nova-ed by a prepared party.
5. Hordes. With Minions, you can have the PCs kick butt at any level. You can send two dozen zombies at them...they'll knock them down fast, but at the same time, they're very dangerous to the PCs. They still hit like a normal monster...they still have the defenses of a normal monster. But you just remove them with a single hit. It's exactly like Aragorn defending Frodo at Amon Hen.
6. Monsters aren't made the same way as a PC is. They're likely to die in an hour, so why waste lots of time designing them? They can 'break the rules' to give a challenge to the PCs. Although 3e was interesting in having this as a basis to the game, I discovered after running it for a few years that I really hated this aspect. It was too complex with no payoff.
7. 4e really encourages the use of environmental effects, terrain features and hazards as part of the game. By explicitly including these makes the set piece battles more interesting.
8. Beholders. By far, the 4e Beholders are the best of any D&D incarnation. Unfortunately I never got to sick one on my PCs! I did at least sick a Death Tyrant on them though.
9. No more scry-buff-teleport. In fact, no more buffing pre-combat. To some this may be a bad thing, but usually it made a combat a non-event...and more often than not that combat was supposed to be a big set-piece Epic boss fight.
10. Combats themselves are more interesting. There is movement. There are tactics. Each class brings something interesting to combats. Fighters just don't roll to hit then roll to damage and then you're onto the next player. There is a dark side to this though (and I'll discuss it later).
11. No fudging needed. The PCs are resilient (more on that later). You can really throw the book at them and they can handle it. I never felt that I had to fudge the dice to eliminate a single bad roll, even at low levels. I often ended up doing this in earlier editions.
12. Some of the 4e books are the best of their kind. The DMG and especially DMG2 are great. Underdark is great. The Plane Below is mostly great.
What I Don't Like About 4e
1. Combats take too bloody long at higher levels. I've already discussed this, but it's a major problem and I'm not sure what can be done about it. At the Heroic tier it's not so bad, but still combats usually probably would take 30-45 minutes. It's tough to have a heroic, pulpy feel to a game when everytime the 'action' occurs it takes over an hour to resolve. This isn't something that 3e features either though, so you really have to go back to 2e or earlier, or even go to something like Savage Worlds.
2. It's hard to scare the players. In earlier editions you had level drain. You had poison that either killed instantly or drained your abilities. You had dragons that did massive amounts of damage. I can't think of too many times where the players were going "Oh...crap." when faced with any monster.
3. It's really hard to kill the PCs, especially at higher levels. It's almost impossible to get the PCs to negative 1/2 max hp, and monsters don't do extra damage on critical hits. Typed damage isn't as dangerous as untyped damage, because it's easy for the PCs to get resistances.
4. Published adventures are very combat heavy. Now, early adventures were similar, but there was a bigger exploration component. That seems to be missing now. 4e is very good at doing dramatic scenes, but not so good at mundane exploration. Although you do level up faster than 1e or 2e, it would still take 3 years at our current progress to go from 1-30. It wouldn't be so bad if combats took less time to resolve, especially since most of the time, the PCs are pretty much guaranteed to win anyways.
5. The system is different enough that you cannot really convert earlier edition's modules. This is largely due to the fact that 4e is encounter based, whereas earlier editions are attrition based.
6. Magic Items are dull. They aren't very powerful individually, yet en masse they could unbalance the system (which is why there are restrictions on the use of magic item daily powers). They also add to the complexity of choice for power usage which doesn't payoff their problems.
7. The economy of treasure goes exponential...when you're in epic tier you're getting treasure amounts of 100,000+ gp but the only thing you can really do with that is make more magic items. Which are dull. It costs massive amounts of money for an ioun stone that only makes it so you don't need to breathe or eat. 3e had this problem too.
8. WAY too many situational modifiers. +1 damage for being bloodied. +2 damage for being in an aura. -2 for this. +2 for that. For hit rolls it's significant, but often the situational modifiers (especially on damage) don't justify the added complexity.
9. 4e doesn't really handle the 'dungeon crawl' well. Dungeon crawls featured wandering monsters...lots of rooms with monsters. In 4e it'd take you a lifetime to actually run Undermountain.
10. Epic Tier adventuring is significantly different, but no good advice or adventures that feature it. It also almost exclusively seems to emphasize extra-planar stuff.
11. The Disease Track is a great mechanic, but then they screw it up by making it nearly trivial, especially at anything higher than 10th level, to use Cure Disease to remove it.
12. Even if you do manage to kill a PC, it's pretty trivial to bring them back. Raise dead arrives at only 8th level, and it really isn't that expensive to raise someone. Even then, they only take a temporary -1 to hit rolls and other die rolls for 6 encounters I think.
13. Power Cards seem to reduce creativity. On the other hand, usually PCs were just swinging and hitting anyways. Powers make 'creativity' the normal circumstance. PCs can do cool stuff by default now. Part of the problem is that the PC knows what happens when he uses one of his powers, but trying to drape a tapestry over an opponent's head has an uncertain effect.
All in all, I really like the 4e system, especially as far as the monsters and classes go, in general. However, the length of combats is a killer in my opinion.