2. The notion that "god" has no partners and yet Archangel Gabriel and Mohammed were both needed to create the Qur'an. In theory, that makes them partners as both have an "inside scoop" from the boss. Also, Muslims asked Muhammed endless questions about how to behave and he did not tell them to ask "god" directly, but offered his own opinion. Again... that does tend to make him a rather vocal "partner".
3. Belief in the virgin birth, while maintaining that the son of Mary is not also the son of God. If it was a virgin birth then legally speaking it does not leave a lot of room for who the "daddy" was.
4. There is a fair amount of evidence in both the Qur'an and the Sunnah that Muhammed suffered from quite possibly more than one form of mental illness.
5. A personal reason: I personally detect little joy from Muslims, almost never in fact, in person or in conversations on the Internet. I am not entirely sure why this is, but Islam does not seem to be fun -- at all, as it is taken so drearily seriously by the believers. My personal feeling is that Muslims take themselves far too seriously, to the point where it perhaps becomes unhealthy. Further to this the sole reason for proselytizing would seem to boil down to "misery loves company".