I think this release is great in that we have three races that ARE similar, but still have some depth in terms of differences in research and ship function. I definately agree that completely different play styles/units/factions are the difference between 10/10 and 8/10 games.
I'm not really sure how many changes they could realistically make to the existing three races without essentially completely stripping them down and starting from the bare bones. Ideally, I would have liked to have seen the Vasari be along the lines of a super-advanced, Protoss-esq faction with emphasized tech advantages. The Advent, not really too sure, maybe could have been a very nomadic race as in virtually all of their structures are mobile/could jump around systems. That could have worked with their backstory also. TEC I think are great being the stereotypical human space faring race with rugged but functional ships.
I think it would be interesting to see the race that the Vasari fleeing from being some sort of biological space faring race, ala Zergish. That would be something very different that players would notice instantly both visually and in gameplay.
I'm not trying to make it seem like the devs should be cloning Starcraft or anythihng, I was just using the Protoss and Zerg examples as they are the best examples of different play styles out there that I can think of.
Of course, the more different you make the races, the harder it is to balance them out against one another. BUT it can be done, and while I'm really happy with Sins, I'm even more excited for future expansions/sequels now that the team has this under their belt

Cheers