yea, i find the later skills aren't worth using unless you have a good rune to use it with... i'm only lev 15, but i'm still using a skill/rune i got in lev 10 when i have new skills to use
also, joining someone else's game doesn't save your campaign progress.. you have to continue your own campaign to make progress that you can hop back into later. Joining a party that was at Diablo means that you gotta start from scratch if you have no party to join. Although there are tons of public games you can join. But yea, i agree with jai, it takes part of the fun out of playing a game like this if you're hopping into it just to power level and skip the story