1. They're very short-term thinkers. Brilliant ones, but still short-term. They come up with amazing inventions and technology and cultures and then don't have any idea how they're going to get along with each other and the earth itself.
2. They're often fairly selfish. They spend their lives chasing money and material goods, with the exception of a minority. They put a lot of value on what others think of them, and they go to great, sometimes ludicrous lengths to meet arbitrary cultural standards for things like beauty and masculinity.
3. They're very ambitious. This leads both to great successes and great failures, such as all our technology, and all our wars.
4. They're probably a lot like we (the aliens) are. I mean, every race will have conflicts of some kind, and every race will want material goods in some quantity, and there will be evil people anywhere.
I guess I'm not thoroughly disillusioned like lots of people here are. Does everyone really think that the world would be better if humans just all died out instantly, right now? Do _you_ want to die, right now? Seriously guys.