Money. Pretty much everything to do with money.
The biggest problem is that he honest-to-goodness just doesn't grasp the concept. Which is more frustrating than anything, because I can talk until I'm blue in the face and it just doesn't sink in. He doesn't get how it adds up to spend a little bit here and there. He doesn't get that it all goes into the same fund, so when we spend too much on everyday stuff, we have to juggle bills - or vice versa: if I've already paid all the bills, we may have to wait on everyday stuff.
Oh yeah, and there's "his" money and "our" money (meaning what I earn). So I work 40+ hours a week, plus side jobs for web design and graphics, and that's to pay the bills. He does one odd job of web design or computer maintenance, and that's ALL his play money.
Unless I fight him for it.
He also has a BAD habit of spending money before we get it...either in fact or in his mind. There's a couple problems...one, he will "borrow" money from checking (bills) or savings to get something he needs RIGHT NOW when he knows that money is coming, and then promptly spends the money when it comes in too so the accounts never get paid back (and then it's my problem to figure out how to pay for everything we need). Or, if he doesn't do that, he will bombard me with "We could spend it on x, y, z, p, d, q, f, j, s" when I'm busy with other stuff. I'll give a non-committal reply, which he will later use as a "But you SAID we could get this!"
He's getting better, though. Honestly, he is. Because most of the time he doesn't realize what he does, so when I point it out to him about a million times he starts to see it.