Christians - followed scriptures that had been altered, and if they still did not like it, would alter it even more.
I still think that. But for the most part, it seems like they're willing to admit it. 'Oh yes, er, that part of the Bible about stoning your child who curses at you - that was for that time and culture' and 'Oh the Old Testament - that is the old law, Jebus came to give us the NEW law.' Even though I'm not Muslim, also I still think that the stuff the Shia do in Muharram is weird. It is. But the difference is that I no longer think they are engaging in sinful behavior for it and I also recognise that Sunnis have some weird stuff of their own (though not big on ceremony, were we).
Stereotypes I had
Atheists were arrogant or they just didn't hear the message in the right way yet, depending on who they were and how they approached things
Gay people didn't have the support they needed from the 'true believers' to struggle with their jihad, but I also had a problem with the gay bits in Islam b/c I just believed in my heart that people are born gay, they don't make themselves gay and I wasn't sure about saying 'Ok this is your jihad, and you've still got to marry someone of the opposite gender and just make a go of it.'
Non hijabis are slaves of the West who can't hack the struggles of the straight path. Of course, I only started thinking this once I myself started wearing hijab.
Niqabis are bitchy. I still think this one to an extent, but I admit it is based on my own personal experiences in life.
Apostates are weak people who were seduced by the easiness of the secular lifestyle and they are ignorant of Islam, and if they really knew Islam and what was waiting for them in the eternal hellfires, they'd come back today.
Most Muslims are racist against one another and against others. I still do believe this, again based on personal experiences only.