Black Mage: the major battle system of S1? Yuck - that was my biggest reason for choosing S2. S1 was just a complicated game of "rock, paper, scissors." S2, S3, and S4 actually required some military strategy("let's see, send these units hear, then have that one attack first, then surround that guy and then use magic on that one..." etc.).
DMKA: it's raved about by a small group, but it's still largely underrated. It's not a coincidence that Suikoden is proclaimed the best RPG series by just about everybody that's played it.
Daniel: A lot of people start late in the series. I actually played S2 first. It doesn't affect the enjoyment of earlier games. They can be enjoyed independently.