Thank you for the replies. I agree with most of what you all say regarding practitioners from other MA, in fact most of my best students have done some other form of MA (especialy Tae Kwon Do) before Aikido.
Also, I apologise if I led you all to believe that I felt that people from other MA were "aggressive" in some form as a result of their training. I myself trained in some Karate and Judo, before Aikido also. I guess "aggressive" was the best word I could use to describe a very abstract feeling that I get from some of those who come from other styles (these, however are in the vast minority, most are very cool).
You have also made me realise that I should explain myself further.
I think PeterR hit the nail on the head in referring to those who have "limited" knowledge of their previous arts. I think this tends to create confusion in some people as their mind/body coordination is being redesigned by different MA at the same time.
The problem I think arises when these folks come to Aikido without "emptying their glass" so to speak and try to make Aikido conform with what they've learnt (or are learning) in other MA and not approaching with a totally open mind to Aikido practice
I guess the hidden question here is whether MA who come to Aikido to show how good they are at some other style or to get an ego trip should even be in an Aikido class, and doesn't the instructor have a duty to protect the other students from those who have this desire to show off at their Uke's expense?
Please keep the comments coming, they are really helpful.