As in D1, what made me keep playing D2 so long after beating the game some 100 times or more was again PvP. The same happens now with D3. I have now 6 lvl 60 chars, and that means I went through the entire game story at least 24 times (6 chars x 4 difficulty levels)! Besides, I made uncountable farming and key searching runs (not so many as some people, since I work and have a family). The purpose of all that has been (at least for me) essentially the search for better items for PvP.
Unhappily, the present version of the game is very limited in what concerns PvP. There is only FFA (Free for all) and the classes are very unbalanced. Only top geared chars have a chance not to die in one shot and PvM builds are mostly bad or totally useless for PvP. For instance, my best PvM char is a double tornado Barb, that farms at MP10 without problems, but absolutely sucks at PvP. By now my DH is the only one really fun for PvP. I have a PvP barb that still sucks and my Monk is getting better, but a well built and top equipped WD has no match (I have one, but very poorly equipped). I think Blizzard will have to do some investment here if they want the game to be a long term success as D1 and D2.
Do you think such improvements are to expect?