I play on lots of different maps and sizes, but I usually stick to small, medium, and large maps on Standard speed. Sometimes I play on Epic, but never Marathon. Besides being mind-numbingly slow, I also think Marathon is an exploit.
I wish that Firaxis would re-implement religion to make it more than a virus that causes diplomacy modifiers ("You have become infected with the wrong virus! I shall now declare war on you!"), rather than adding half-assed features like vassal states and corporations. I don't think anyone wanted vassal states to be a strategic element to the game, but that's what it ended up as. What was wrong with the model in Alpha Centauri? Oh, I see; there were no drawbacks to capitulation. We can't have that! It was too fun. So, now, when your opponents capitulate, you end up with worthless vassals that can't wage war worth crap, and you dare not donate to them any technology, for fear that they give it away to your worst enemy! But if you don't accept their capitulation, your weakened opponent will almost certainly capitulate to a different civilization. Gah. Frustrating. This is not fun, though it does add more strategic depth to the game. For all of it's annoyances, I still play vassal states on, because I appreciate the strategic depth (which is sorely lacking in Civ IV). I guess you could say that I think the game is slightly worse for not having any strategic depth than it is for having a really annoying feature.
Corporations are the same deal. Your first instinct is to spread all your corporations to all your cities, but after your first game, you realize the folly of that move. So, then I realized it was meant as yet another strategic element. Crap. I thought corporations would be something desirable, but I'm finding that they're sometimes more of a hassle than they're worth. I mean, sure, the strategy of deciding which corporation should be added where is kind of fun, but I was really thinking that corporations wouldn't be so heavy-handed and punitive in their implementation. Oh well. But, like vassal states, I appreciate the effort to make the game more strategic.
Personally, I wouldn't have implemented these features in the same way. Instead, I would have made the civics a bit more strategic. I would have made the units a bit more diverse. I would have changed the combat system so that waves of low-tech units wouldn't have a prayer against a single high-tech unit. But, then, I liked Alpha Centauri, and I thought that Civ IV was going to steal more concepts -- and leave them untouched -- than Firaxis did. I'm still waiting for some of the better Alpha Centauri features to be implemented in Civ IV. Maybe in the next expansion. Ha.
So, in conclusion, I think many people complaining about vassal states, corporations, etc are annoyed that these features ended up being strategic elements, rather than something like religion, which is bland, boring, and easily abused. Many of us, when we heard about vassal states, thought that some of the tedium required in conquering the world would be decreased; instead, it turns out that we have to deal with a further annoyance in a game that was supposed to remove all annoying aspects of the previous game! Consider this for a moment.
Okay, I've gone on for long enough; and, for those snarky posters who feel the need to tell me to play AC if I'm such a big fan of it: I still do, occasionally. It's a welcome break from the annoyances of vassal states, inanity of religion, and silliness of culture. But the AI is crap. Long live the Civ IV 'Better AI' mod! Blake truly is our savior, and one of the few reasons that I'd rank Civ IV over AC.