expansion might mean more defense needed, but typically it just means that you need to figure out where you need to defend. If you defend a lot of cities heavily, then you probably are wasting hammers or gold. If you have a bunch of cities, then it's easier to produce units and allocate them to the right spots. If you happen to lose one, then you can typically try to get it back by producing lots of units out of your superior number of cities, or you can just not worry about it if you are really dominating in expansion.
As far as REX being a problem for CivRev, I don't know. Sid wanted to make a game that was faster paced, and he explictly chose to leave out corruption to help streamline the game, which I think was a good call.
There are multiple ways to win the game, and you don't always have to expand a whole lot to do so, but if you do not, this typically means that you are going for a quick kill and playing aggressively. The most consistent way to win the game is to rapidly expand, get a larger empire, and dominate in tech or military.
This is why the Chinese, Americans, and Romans are really powerful, because they have schemes that make it so much easier to expand with on top of already good bonuses. Lots of civs just can keep up. It's important to try and expand with all civs so you have a more flexible and realistic shot to keep up with civs that have an easier time expanding, but the Germans aren't going to do well playing an expansonist game against the Chinese and hope to have a shot in the modern era if both are expanding well. The Chinese would kill in tech, and the Germans would have needed to strike well before the Modern Era.
I would be interested to see some kind of corruption/pollution (I guess, I'm not very familiar with the PC Civ games), to help combat this, and maybe provide a little more depth. I think the game has a little room for slowing it down some, but not by much. If the average game length goes up by an hour, then it'd be a little too much.
Rapid Expansion hasn't ruined the game, but it has taken a little bit of the variety out of it. It still takes a lot of skill to do it well, but it could be improved upon somehow. The way I have won probably 50% of my MP games was just not lose to an early rush, and then build more cities than the other guy. Lots of players just don't understand expansion still, and try to win with 2-4 cities all with libraries type of strategy. This doesn't work. A rapid expander will win 95% of those games.
