Pretty much anything, except other contextural ads (like YPN, Clicksor for example), adult ads or extreme amounts of popups.
Nothing specific, just thinking of diversifying my ads and adding more products, I just don't want to piss google off.
Nope, they're not allowed to obstruct browsing or pop more than 5 times per user session. I see several sites per day that breaks the 5 times per user session bit.