Most of the complaints I've seen about 300 fall into one of two camps. The first is the 'reading to much into it' camp, where they claim they're pushing a political message about Iraq/Iran. (Although who's playing WHICH side depends on who you ask, I've seen it both ways)
The other camp tends to be the 'That's not realistic!' camp, which tends to be based around the idea that a movie based around a historic event should be made to look/follow that event as close as possible. I see this one more around the historians, armchair or otherwise.