I could be wrong here, and this is just an opinion mind you, but StarCraft, also by Blizzard was one heck of a game that had very different races, the Zerg being my favorite. But that said, Sins has in my mind a wonderful difference in that the races although being different, aren't TOO different, by that i mean going from one race to another requiring me to completely re-learn how to play.
But yes, difference and more races would be nice in my book, but maybe not too many. And the importance will be to have the racial differences be strong enough to play out interestingly.
Say a race of Diplomats? That attack with culture and intrigue and behind the scenes backstabbing/politics/influence buying without ever raising a fleet above small size. Perhaps they can't raise a fleet above small size. This could allow players who are very much into that kind of thing to play, and win in scenarios against the vasari, or tec or advent. Perhaps the influence could "turn" a forthcoming raid/invasion on its head and stop it. Something the other factions do not have the ability to even attempt.
Or Trade race, where trade ships and purchasing/selling to the market, or to each race is the prime point. Where fleets of tradeships are "guarded" by the advent or vasari, or tec, depending on what is being shipped and where. This wouldnt mean not losing any ships at all, but the "guards" or the pirate "buyout" could mean a mostly hassle free shipment of goods from one place to another. Perhaps in this scenario, the Trade Repulblic would only "win" in alliance with the faction that they pin most of their trade to, while "starving" the others, but that might be a decision late game after playing "all of the factions" in the beginning to determine which way the war is going.
Biological race, where "grown" ships (perhaps in the manner of Zerg or perhaps not) give them an edge in certain aspects of battle. Perhaps Giant ameoba that draw /absorb weapons fire from attacking ships and ultimately dies, but gives the small more fragile fleet the chance to sneak in and either "corrupt" planetmodules or ships, "infect" them so to say, rendering them either slow decay and erratic behavior or converting them, i would prefer the former as we already have a conversion ability in the advent, and it would be nice to see some difference from that. But with enough infected ships, modules, planets it could be a way where any of the other factions have the ONLY choice of losing vast amounts of low end ships or targets in order to overcome the relative at a distance killing of these infectors. This could mean warfare with this race in a more "indirect" approach, instead of stand off fight to the death weapons fest that we have now. It could bring a new approach and tactics of course, and by doing so could make the game more interesting.
What im saying here, is not just to introduce ANY race, or random-quick drawn thrown into the game kinds of races, but races that have distinct and very obvious and "Playable" differences. The ones i note above are only ideas, but my thrust here is to create a different kind of "play approach" when playing them.
Anyway, just some ideas and thoughts, take care and keep modding!!!