It is looking more and more like a military simulation, not a Civ game. All I keep hearing about is huge armies, large battles, ranged combat, city defense -- not economic models, research programs, or space races. The market is full of war games, but there is only one Civ franchise. I'm afraid the focus has been changed.
This is about my only big one, but I'll expand on perhaps things I see differently or am just more worried about:
The actual gameplay will be more limited, simplified, and "easier" without leaving the player as much opportunity for replayability and enjoyment. And, in a roundabout way, all the talk about combat etc... just serves to try to HIDE this.
One problem here, common to many game developers of course, is the focus on updating "eye-candy," graphics and stuff etc... and not improving gameplay. 3D animations, talking leaderheads, whatever, all that stuff I would not put as a priority, the things that serve virtually no function, but will take up lots of memory and programming and processing time and make for good advertisements and promotions.
Then, you have the major example of the CivRevolution game, which plays extremely different from any real civ franchise game, and isn't what most longtime fans are probably looking for in civ 5. In an attempt to open up for new demographics or new gameplay or whatever, civ 5 could end up removing a lot of features (see: religion, being another one) to make the game simpler and shorter for new players.
So when we get back around to military, what are the concerns?
-One-unit-per-tile, and everything implied about that, including that the size of maps, and armies, and civilizations will be limited. People say civ IV is "small" compared to civ III, where maps and games could be huge and epic - well, they haven't seen anything yet, because I've seen nothing to assuage my worries that this will be a much larger problem with civ 5. This is essentially what I quoted, that civilization is not a wargame, and the more people beg for "highly tactical unit battles (insert 'real-time' or 'reference another game series' here)" and stuff, that's actually what we will get.
-The AI, even through no fault of the developers, because it is AI after all, will just suck. Part of this cannot be helped and has to be accepted. However, features may introduced or designed in a way that lead to further brokenness - with new tactics and gameplay and most important to player-AI interaction, war and diplomacy, the AI could either always be hopelessly incompetent, or the game will be built with so many of those "cheats" that civ players in general are always begging them to get rid of, and rightly so, since they can be annoying. And this is what kills replayability above all. Civilization 5 will simply NOT succeed as a multiplayer game for the casual gamer, I can say it now, and eat one of my TF2 hats if I'm wrong. Because civilization games take too long and are too complicated to play multiplayer for the average complete newbie - and that's ok, they work for fans who are committed and of the course the epic single player is the core of it. But if things are simplified, streamlined, dumbed-down, then that core of the game and its lasting appeal is what gets detracted.
Again, as I've said elsewhere, I do hope to enjoy the game and a lot of new things sound cool and promising...hex tiles for instance

but these are the worries I would want to get through.