Lack of sustainable content, as the OP pointed out, is the major problem for me.
It isn't even hard to come up with ideas for it.
One example of what this game could use is a "survival mode", like Left 4 Dead had, and Borderlands eventually got. Where wave after wave of enemies comes at you, and you are simply trying to last as long as possible. Wave 3 would have a boss, Wave 4 would have a stronger boss, Wave 5 might have 2 bosses, etc. Best times would be saved for each player and shown for all to beat. You can find rarer items as you progress farther, with great items only dropping once the difficulty has gotten crazy (a wave of 3 red tyrants, or whatever).
If you somehow haven't seen this in Left 4 Dead already, here's an example:
YouTube - Left 4 Dead - Survival Mode Gold Achievement No Mercy Generator Room
And Borderlands (kinda just looks like random enemies, but its more powerful waves each round):
YouTube - Borderlands PC - Bandit Arena - Final Round - Part 1