Well - in the past I've been more of a shooty type guy, as I played Tau in 40k, Cygnar in WM, but Trollbloods in Hordes, which are the opposite of shooty. If I had to say what draws me most to a faction it's mostly lore and look, but I also want something to be fun of course.
I guess if I had to pick a playstyle, it would be diverse. One thing I came to hate about Tau was that it was a one trick pony, and playing Empire in WHFB was nice because they were flexible.
As one of you said, it's not like you can't collect other armies and the system apparently lets you mix and match, which is nice.