He doesn't have to pay any income taxes because he has no steady income. He retired in 2002.
I think he might have a lot of controversial investments or offshore accounts. He probably has complicated trusts and retirement accounts. I'm sure there's some tricky tax avoidance techniques.
He doesn't want to release them because they'll stir up more controversy. If the government is so interested in his finances, Obama can just audit Romney. There's nothing wrong or political with that.
There's no real comprehension of how much damage Romney has DONE TO HIMSELF.
He's done more damage by looking like he's done something tricky, than just being upfront from the get go and addressing it.