I dislike BD greatly, mainly because you attack like you're some sort of fat ogre who's high (specifically the speed at which you attack). But I understand why he didn't like it. Those who have played DD, it's sort of like Devil May Cry 1 & 2, the first is original, but the sequel takes a new look, and you lose that essence which carried the first game high.
Questing / storyline:
The ability to feel like you are achieving something. BD was too linear, and if there were any side quests, which were optional, then I certainly didn't find many at all. The storyline gave me little pleasure, and it wasn't intense enough. In DD it was very loose, but this was ok because of all the side quests and places to explore.
Simplicity:
BD had one too many stats, I for one had no idea how to distribute them, and it still boggles me. It's nice saying experiment, but you get too dragged into the dull running around, and you start not to care.
Player freedom / space :
BD felt too small, to put it simply. The battlelands were good, but that was overdone in my opinion. This is where DD falls slightly, but it still dominates BD easily. The sewers themselves were the big blow, upon replaying a few times, it got incredibly tedious.
Keep it to what the screenshots show in the next game, simplicity and a range of spacious areas to explore, with plenty of side quests. Add good puzzles, which aren't hard, some which require initiative and some skill. i.e. pyramid traps such moving blocks and hidden traps doors and spikes /gg
