I'm back into playing D&D 4th Edition, and as much as it's different from D&D 3rd Edition, it's still a lot of fun. For one, you're much more likely to fight a group of creatures than a single baddy.
For instance, in our last session we invaded a small fortress. Our fifth-level party couldn't agree to attack so finally the Paladin (my character) bluffed his way in, scouted, bluffed his way out, and convinced the Sorcerer (but not the Cleric or Ranger) to go back in. Of course the baddies in the fortress had seen through the Paladin's bluff, due to some unfortunateness, and attacked. So there we were, a Paladin and a Sorcerer fighting fourteen guards, six in melee and eight with bows. We would've done better if the Cleric and Ranger had been in the fight in the first round instead of the fourth round, but we still survived!
For two, you tend to start fights with full hit points and all your spells available.
After our big encounter, we used our healing surges to heal ourselves up to full (a healing surge is one quarter of your total hit points). We then spent five minutes resting (that's called a Short Rest), which refreshed all of our once-per-encounter abilities. And voila, we're ready to delve further into the fortress!
For three, characters are much more stereotypical - Paladins are Paladins, Fighters are Fighters, Wizards are Wizards.
The multiclassing has (basically) gone away and I used to think I'd miss it, but having played a few sessions now I gotta say it's easier to play the stereotypical classes. There is no longer a worry about creating the perfect build to help your party power through encounter after encounter. Unfortunately, to get a complete list of classes you really do need to have both of the Player's Handbooks. And for magical items, you really want to get the Adventurer's Vault. In other words, where in 3rd edition you only needed 3 books for a minimum play experience, in 4th edition I'm suggesting you need 5 books. But then, in 3rd edition people would end up with 7 or more books, so maybe that's not such a big deal.