I have to say that I am not a fan of 4th. Ed. I think the combat heavy focus in the books (compare player handbooks of 3.5 and this and tell me how many pages have a combat dedication) just leads new DMs down a dark path to rail-roading. Now this may seem like a bit of a leap but let me explain. You need to plan encounters thoroughly in 4th ed. There is no escaping this fact because of the greater amount of 'tactics' and wargaming involved in how the combat mechanic works. This means a DM has already invested an awful lot of time into something that, with a good story and an intelligent party, could be avoided completely. However, in my experience, what ends up happening is that these new DMs find they can make the exact combat encounter they want time after time...so they want every single one to be played. Ultimately this will cut into the story and player options.
Plan? Encounters? Although I might agree that WoTC's approach to D&D is far from most gamers' ideals, I don't think planning encounters is an issue, you just pick monsters in the XP range of the players and have at it. I don't even look it up in advance.
Like malyss, I quite like the fact of not having to spend half an hour putting stat on monster. Most of them were created by wizards anyway, why would they be bound by the rules that apply to humanoids?
I think what makes it 4E good, can also be one of its main problem. Encounters are usually fun and quick to make, but in a story-oriented game, they need meaning.
I'm playing two 4E games right now, both of them I'm both player and DM as we alternate. In one, we're empty shells of characters who fight monsters after monsters. It was what we agreed to do from from the get-go and we're having fun seeing how unkillable our characters are.
In the other, all our characters have backgrounds with various relationship and we have our personal goals. It's quite fun when we further along our own goals or the others, but then you have one of the DM go: "A dwarf arrives in the room (interrupting our RP), he's quite beaten up. He needs help fighting some monsters that appeared in ruins about a day from here!"