I absolutey love this game! I have only done one play through so far, so havent even hit the level cap, but yet I know I'll play this again and again.
The great thing that I find is that you can quite easily play the game on your own with it being fun, looking at all the loot, building your characters stats etc... :). Of course playing with others the mobs drop better loot and that loot is all for you, which is good for the little greedy gremlin inside us all.
One of the exciting things is getting to the level of a skill and trying it out, there's a pretty nice variety. You can chose to focus or spread them out still feel pretty powerful.
(Not gonna go into too much detail with the level skills, dont want to ruin it for you lol) :D