One thing that I've found with the alt-coins that caught on were:
1) They have a catchy name (Digitalcoin, worldcoin, etc have much better names than some of the other ones)
2) There is something unique about them (primecoin has the angle of being 'more useful')
3) They have developers that are involved with the community and are adding new reasons to adopt the coin over other ones
There seems to be a lot of choice right now, but being a little observant you will definitely see some coins just appearing more valuable than others. Along with general economics and scarcity and all that stuff