honestly i found that both d2 and d3 got pros and cons.. for me, i still prefer to classic way of learning skill with synergies like in d2 but skill system in d3 also not bad..

for storyline i prefer d3.. d2 storyline more straightforward.. but d3 give u a few key factors that could become spoilers if u didnt finish it yet.. no spoiler for d2 tho.. "so what's the story in d2?" "well, u go act 3 u kill mephisto, u go act 4 u kill diablo.. thats it i guess"

single player in d3? just create ur own game and dont let anyone to enter, there ur single player.. or u mean offline play? i think there are a few factors blizzard wants it online.. one is DRM.. and the other factor is d2 itself.. d2 famous with their battle net play with ladders.. but no ladders in d3.. maybe can suggest to blizzard to add a ladder for hardcore chars.. and another one is to encourage coop play..

graphics? cant expect much as blizzard is not a graphic whore game.. (unless for cinematics of course) see WoW.. but it really improves from d3.. in terms of geometry and skill animations.. have u seen skill animation for monk?

gameplay too short? have u been to inferno yet?

boss fight? i think d3 better with all the phases when fight main boss..

yeah, i can see that they will release expansion for d3.. lets wait for that.. hope they will be a comeback for paladin