Sequel where nothing is changed = Rehash! OMG stagnant gameplay!
Sequel where gameplay is changed = OMG my series is ruined forever. Look at all these new people playing my game!
Thing is, there should be a balance. Mass Effect is one of the better examples. It was stated to be a trilogy from the get go, so you get the whole experience by getting all 3 games. The first one is released and eventually a fanbase appears. Those fans loved the game for reasons, sometimes different reasons. Still, the game had its fair share of criticism; inventory is a clutter, planet exploration is repetitive, combat could be better. Second game is released: instead of fixing the inventory or improving planet exploration, they both get on the chopping block. Combat is kinda improved, but then the RPG aspect of it almost disappeared. Combat no longer take place in the normal gameplay areas, but areas specifically designed for combat. So basically, the world gets disconnected, and you talk to people to start your "missions", which are now basically Gears of War missions with a couple of dialogues in-between, making level design quite different. Instead of feeling like playing an RPG in which you shoot enemies, you feel like playing an RPG with shooter missions. So what happens, is we get two similar, yet very different games, there were some
fundamental changes. And as much as it's a good idea to change things a bit, you have to keep in mind your fanbase likes your game for reasons, and if you ditch some of the reasons, they're gonna get angry. Personally, I thought Mass Effect 1 was a good game with plenty of potential. I thought planet exploration was a unique feature which offered those "sci-fi moments". As much as some planets were repetitive and all, I found them a nice touch that added immersion and atmosphere. Sure they could be better, but imagine if it was better! I also loved how the combat was implemented in the first, it was not disconnected from the game, you were not playing 2 different kinds of game. These are elements that made me like Mass Effect 1, you take them out, and while I may still like the end product, it's still what made me like the game in the first place, with that gone, it's not as appealing. And those are not the only problems I got with the game, like the feel was from more old-school sci-fi to campy Hollywood stuff. And when you make a trilogy of games, with a story that follows, there's a problem if a fan of the first can't enjoy, or not as much, the other ones. It's an hyperbole, but imagine if Halo 2 was a racing game and the 3rd a fighting game. Doesn't make sense, does it? It's just normal to expect certain things from sequels.
Sometimes the developers feel to ditch and replace, when all they could do was improve, while still innovating. It's like with Splinter Cell Conviction. Fans praise the better AI, when it's only a granted, it's not the original design that made the AI less good, it was the technology. Or the more open environments, again they could have improved on the original design and made the environments more open. In fact they got a bit more open with each game. Splinter Cell Conviction is an offensive one. I mean, you have a legion of fans who loved the original games, and then you change the WHOLE thing. You know, when you make a sequel, it has to earn the name.