As someone pointed out- all RTS are 4X; its not a hard criteria; the real yet sadly fairly abstract difference between an RTS and a typical turn based empire game is two fold.
1. Scale *Sins solves this adequately

2. Role Play
Believe it or not the advantage of most 4X games in terms of their form of fun as opposed to most RTS is that you end role playing your species or your people, you research as such and create mini-stories around typical game events/wars/etc.
That doesn't happen in RTS because there is not enough time to become emotionally invested, and most focus on tactics as opposed to nuances between faction. That doesn't mean people don't enjoy the stories of course, and pick races based on favorites, its just that the RTS empire is not personalized to you.
Sins does not breach this barrier, but it does come closer than any other RTS to create the same role-play experience that space empire (or land) 4X games do.
-Also; Culture once upon a time did take over enemy planets; ultimately it was too good; especially against the AI, and most people did not like it, and there was a million ways to change it... Thus they weakened it and made it a nice defensive measure, but offensively it isn't a full strategy all on its own.
Now in some ways thats good; it promotes combined arms tactics; but in other ways it makes playing the game as say a role playing 'peaceful' people; very difficult.
What you guys should try to keep in mind is that StarDock (and Ironclad by proxy) create games that don't stop being made... they get continually support; with a game like Sins 'production' is never really over.
I'm certain that diplomacy will be getting a big look at for the first big content add on... and with that probablly culture. So i wouldn't sell off such features, the Devs just need more time to digest their own creation, especially in response to the now birthed community.
Basically- suggest things, its great! But I don't think you should expect culture to just take over planets; we have been down that route, and it may come back, but it won't be easy or simple I'm sure.