To me, the general feature list for the expansion is here:
http://www.arcengames.com/aiwar_zenithremnant.php The bullets don't mention it, but the narrative text talks a lot about how much more varied it makes... everything.
It's certainly not expected that everyone have the expansion, or that everyone have an encyclopedic knowledge of the game in general, or that everyone be using the latest beta. Aside from the strategy discussion subforum and a few other subforums, most people on this forum are using the latest beta (with or without expansion), though, simply because that's why they are here and what they are here to discuss. Not that there's anything wrong with being here and not being on latest beta, don't misunderstand me, but that's just how the population tends to be in the majority.
Feedback on the base game is very valid, and it's a great point, but if there's something that was already "solved" in an expansion, then it's not something I'm going to be keen for discussing in the base game itself. Again, please don't misunderstand: we fix bugs for everyone, the AI logic improvements are for everyone, interface improvements are for everyone, etc. But if it's a "I wish there was more to do" type question in particular... see my comments above. The free DLC is bite-size chunks of that, where the expansions are the fire hydrant.
Regarding the forum structure, it's that way for a reason. When we have 5 expansions in another four years, if we do indeed hit that point, it's going to be even more of a stew of players with various expansions, for sure. And honestly, that's okay. Strategy suggestions are always a bit tricky in general, because you never know what options a player has in a given scenario based on what their bonus ships are, etc. So discussion tends to be "use X or Y or Z, or if you have the expansion you can also use Z1 or Y1" or things along those lines.
Honestly, the expansion has been out in prerelease form for about 7 months now, and in full form for almost 5, and this has never been raised as an issue other than in this thread. My feeling is that it's working pretty well -- and the design of how all this would work was something that we certainly put a lot of thought into, and that I feel works better than what other RTS games are doing with their own designs.