I love DnD, I really do. However, let's be honest, not a lot of roleplaying actually goes on in it. You roll up your character. Your class tells you how you kill monsters, and hence which attributes you use. If you're a Fighter, Barbarian or Paladin, you pummel them. If you're a Sorcerer or Wizard, you cast magic missile at them. If you're a rogue, you sneak up from behind and stick them with a knife. If you're a Ranger or Druid your animal companion rips their throat out. There are two support classes, Clerics to heal and Bards to buff. That's about it.
You then choose skills. There are about four skills you actually use. Spot/Move silently/Heal/Bluff. That is about it.
You then decide feats. Feats give you handy ways to increase your killing or surviving powers. Either you take a feat like Cleave, allowing you to kill another monster after this one, or a feat like Lightning Reflexes, allowing you to survive a trap.
You then fall into a pattern which goes something like this: Get plot hook, go to dungeon, empty first floor, loot corpse, kill monster, kill monster, find treasure, go to level two, kill slightly harder monster.................find dungeon boss, kill dungeon boss, go back to town, hand in dungeon boss's ear to plot hook giver, get new plot hook...repeat.
I'm kidding to an extent. I have played brilliantly scripted campaigns with exellent storylines and superb villains. However, a lot of them can still be broken down to this basic formula.
That said, put a D20 in my hand and tell me my attack bonus and damage, and that dice will be rolled before you can blink.
So, if you don't play DnD for the story, what do you play it for? Partly it's a social thing. Six or seven people sit around a table making endless jokes and snide comments for four hours. Roleplay happens, but in a jokey, lighthearted way. Partly it's as simple as the fact that many-sided dice are so incredibly fun to roll. A D20 is a lovely thing, and rolling it is almost a spiritual experience. One of the reasons why games like Neverwinter Nights can never match up to table-top is simply because of the lack of dice. Finally, DnD allows you to be nothing more than a fighter with a sword hacking his way through an endless swarm of goblins. We have always wanted to do that at some point.
Other systems are much better for roleplaying. Look at the World of Darkness Storyteller System, which encourages playing a role much more. However, none have quite the same effect on me as DnD. I don't know why that is.