Our group is currently playing WFRP2. And my next campaign will be WFRP2, but with a somewhat different setting and feel (Pirates of the Caribbean as The Universe mentioned).
I think that from a combat mechanics standpoint, there is not a lot of difference in the level of crunch between D&D and WFRP. One area that I think it does differ is that D&D has quite a few feats that let you "break" the normal combat rules (things like Cleave or Spring Attack). WFRP doesn't really do that much. Almost all the talents modify things within the existing combat mechanics, which I think streamlines things a bit.
The major difference is with character generation, development and advancement. As has been mentioned, the random character generation is very speedy (for a starting character) but offers few choices for the player. In many ways I find this a refreshing break from the way we've played D&D the last few years where you pick every aspect of your character from the ground up (including that we've used Point Buy).
I rather like the "play the hand you're dealt" way things are in WFRP. Yes, your starting career may suck. But it's very gratifying to rise from being a peasant in the street to being a bold knight fighting the forces of chaos. The one thing about that that rubs me somewhat the wrong way is how some random careers are so much better than others it can make one PC very much stronger than another at the outset.
We got around that by making an abreviated list of starting professions that ALL suck. Nobody starts out as a Noble, Soldier or Mercenary in our games. EVERYBODY is a lowly career to start with. Our starting party was a Tradesman, a Vagabond, a Messenger and an Agitator. I just rolled up a new character and my starting career choices were Tradesman and Charcoal Burner.
I find this grittiness to be fun because how often is your group really challenged by a pair of goblins? When was the last time that an ordinary spear was viewed as a valuable treasure? Granted this condition doesn't persist all that long into the game but still, the slow power curve assures that the challenges you face will be things that you're blowing through by 5th level in D&D.
And speaking of treasure, it's only but so important in WFRP. Yeah, it's nice to have money and you surely won't have much to start with. But once you DO have it, what will you buy? Your equipment can only be upgraded but so much. You can get the equivalent of a Masterwork sword or shield. You might even get a runemage to ensorcel your breastplate. But you certainly aren't buying Ioun Stones and +4 Flaming Longswords.
What becomes more important in WFRP is who you are. What deeds have you accomplished? Who have you helped or harmed? Prestige and gifts of a ceremonial nature are just as prevalent as your armor and weapons.
I think it's the difference in power level, advancement rate and backdrop that most sets WFRP apart from standard D&D. I have enjoyed the switch. But that is not to say that I would never go back to playing D&D by any means.
Let the peasants come and defy Imperial edict! They will be accused of heresy!
