An example are the Sliver cards....a type of creature that can ONLY be blocked by other Slivers...so if you didn't have that expansion, then you were likely to be taken out easily by someone using those cards. Now, if there were only a few cards, that'd be one thing..but it seemed like every single critter in that expansion was a Sliver...enough, at least, that you could build a deck with an entire army of the things and not a single non-sliver creature in it.