I've done a few of the Burning Crusade instances (Hellfire Citadel, Coilfang Reservoir, one wing of Achindoun, and one wing of Caverns of Time) now, and I have to say that I really like the new style of instances. They are short and sweet, with only 3 or 4 bosses, and very doable in 2 hours or less.
I especially like that most of them have shortcuts back to the beginning after the final boss. That was a very nice touch.
About the only complaint I have is that there doesn't seem like a lot of loot. Usually you get one blue per boss, and given that it's only 5-man, you can usually count on at least one blue going to waste. A couple more blues would not go amiss, I think. I know that I personally only have one dungeon blue so far (though it is a nice one - [Amani Venom-Axe]).
But then again, the instances are short enough that you could easily do several in a week, or even a couple back to back.
The boss fights themselves are fairly creative, especially given the limitations of 5-man content. I especially like the hunter boss who tamed a druid in Coilfang Reservoir. Hilarious! I usually get the job of kiting the bear, which is pretty amusing. Paladin kiting for the win!
I also tried the Caverns of Time instance of Old Hillsbrad. This was very neat, and quite different from most other instances. Thrall needs to learn to slow down, however. He's crazy, Leeroy Jenkins crazy. Also, being so heavily scripted, the instance does seem a bit vulnerable to glitches. During the final fight, Thrall died during the wave of dragonkin, but we survived the wave. He didn't respawn and we couldn't pull the boss, so we were just stuck.
Ah well, so far I really like the instances, and am looking forward to doing a more of them. I can't wait to see what the raid instances are like.