so, its hard to say what you should change without knowing what it needs.

the best I can tell its an 07/08? 2008 KHS XC104 - BikePedia

It appears to be in immaculate shape but...

First off, I can only assume that you're not overflowing with extra cash, hence the craigslist purchase over new. the 07 has alivio components, these are very "upgradeable" but if they're working well now then I would hold off. Id look at the consumables first.

Depending on how hard its been ridden you may want to check the chain for stretching and replace if worn. If the chain is bad then you'll want to evaluate your cassette and replace that as well if worn.

Brake pads could be spent.

Tires?

cables and housings

check your rear shock for slop, forks too.

saddles are very personal, you may need something different if this one doesnt agree w/your butt.

If you dont ride clipless, you should.

So Id say you did pretty well on cl but if you want to spend more the opportunities are right there for ya.

Id suggest riding it as is for now, stash away a little cash, and as things fail replace them then.