First of all, I should tell you that I, myself, hadn't watched The Expendables before watching The Expendables 2, though I don't really think it is the kind of movie that the previous ones should be watched.
It's just the movie to watch when you don't want to think about much. Most of the fun you would have will come from the referrals of previous movies that the cast have been in - some of the tag lines are juuuust hilarious.
Remember the time that all the gaggers were referring to Chuck Norris as undefeatable-one-man-army? It's kinda nice that the screenplay had the sense from the current popular medium.
I wouldn't watch it again, but you'll have great time at the moment if you're a 70's - 90's kid. Enjoy!