Phil, the Chatty DM had a good first post on Critical Hits today, examining the sources of why some parties out there (including mine) are not significantly challenged by the base 4e combat system. Against Level + 0 encounters, my PCs hardly even expend healing surges, let alone drop below 0 hp. Forget about getting to negative bloodied hp or failing 3 Death Saves.
In fact, we've only had two deaths in my campaign. One of those was from friendly fire, and one of those was in the first or second 4e session.
Now, my players have been playing since 1st edition, and to boot we have two Tacticians in the party (both of whom have played Controllers in the campaign). They definitely synergize as Chatty discusses although not to as much of a degree. My PCs (currently 18th level) generally do their characters by themselves, but there is a lot of coordination during the game. Also, the Cleric in the party has definitely focused on healing, and it seems that practically every other character in the party has some sort of healing (warforged), extra saves (the warden), or temporary hit points (the barbarian), insubstantiability (the assassin). The Invoker really has emphasized dazing monsters, which really cuts down on the ability for the monsters to get their good powers in.
Speaking of the Invoker, one thing that is overlooked I think is the fact that most parties have Controllers, who are VERY under-rated in my opinion. Not so much for their offensive abilities, but their defensive abilities in preventing certain monsters from doing their thing.
Another issue is that players in the party have 5 or 6 brains thinking of tactics, or outside of the game, a vast pool of sources for optimizing characters.
Another source of unchallenging encounters is that the one monster per pc really doesn't work well. For 4 or 5 PC's that probably works okay. But I have 6 PCs in my party, and when all six are there, having six monsters doesn't really balance the encounter. You really do need an additional monster.
The problem of course is that this increases the length of the combat encounter.
One other thing that recently occured to me is that I use the '75% hp, +1 damage per 2 levels' house rule in order to decrease the amount of time the encounter takes. I do like the boost in damage that this gives the monsters, and the encounters are certainly shorter, but one aspect of this house rule that is overlooked is that it is therefore easier to drop the first (and subsequent) monsters. This quickly results in a man-power advantage to the PCs, especially if the PCs can take out part of a combo you've set up. I think that the effect of this actually is greater than the 25% less hp...that could end up being one or two less attacks from the PCs since the monsters drop at 0 hp and don't come back.
There are some solutions to this problem.
Mike Shea at Sly Flourish posted recently on "4e Hard Mode" and other ideas for making things tougher on your PCs.
He also has recently posted about harder dragons and also has made several Monster Optimization posts.
But I think that there really needs to be some sort of community for DMs discussing tactics and strategies for monsters, much like the Wizards Character Optimization message board.