"Nudity and Sexually Explicit Material We don't allow content that contains nudity, graphic sex acts, or sexually explicit material. We also don't allow content that drives traffic to commercial pornography sites."
Gratuitous updates are a problem on Android. I find it hard to believe that apps like Wifi Analyzer or Mabilo Ringtones are really being updated 2-3 times a week. Maybe but it seems strange to me. They could just be honestly submitting daily builds or something. Supposedly 1.6 has made improvements to the Market that might nullify the problem (I'm stuck on 1.5 so I wouldn't know firsthand)
For security concerns we did see the fake Bank of America app make it into the Android Market briefly. It's not a widespread problem but it's happened so we can't ignore it.