The first thing I noticed about the DLC (aside from the Sanctuary Guardian beating the everloving crap out of me) is the great increase in difficulty. I can say with confidence that the mobs you find here are more dangerous than any other place you go to in Lordran. I found myself constantly running low on Estus, and I was only in NG. Another thing I've noticed was the aggressiveness of the bosses, how little damage they took, and how comparatively little damage some of them dealt. This helped to really give the fights a cinematic feeling by making the fights longer.
Maps are really pretty, but as far as I can tell aren't that special. I liked the way the Kalameet boss fight was designed, though.
I really like the new items. The guardian armor grants ridiculously high poise and defense at the cost of being freaking heavy; the four pronged plow is hilarious (try its 2 handed R2), the obsidian greatsword's R2 is just too cool. I liked the NPCs, too, although the Marvelous Chester felt too much like a Lautrec clone to me.
I really, really love the arena mode. They really thought of everything. Pity you don't see that many people in deathmatch mode or 2v2...the only complaint I have is the deathmatch system where everyone other than the victim is awarded a kill when someone dies. This promotes people hiding in corners while others duke it out. The system should only reward you when you personally kill someone.