There are a few exceptions to the rule. Dune 2 was completely different from the original, but it single-handedly invented the RTS.
I admire Dragon Age 2 profusely for avoiding creative stagnancy. And I love it for the level of progression it featured in the area of sexual egalitarianism. Overall, it had a clear direction and followed it through.
Same can be said for Far Cry 2, which took the inane shlock of the original and turned it into a highly relevant exposé of socio-political climes in our current day, while simultaneously rendering the player directly complicit in the horrors committed.
If a developer wants to make a sequel to a game that is a significant leap from the original than it shouldn't share the originals name. That's it. If you don't want to make Silent Hill XVIII fit into the canon, theme and gameplay of the original Silent Hill than don't make a Silent Hill game. If you don't want to make Halo 37 about Masterchief shooting lots of aliens than don't make a Halo game. The entire reason you stick that series title at the front of the game's name implies you are making another game in the style and tradition of that earlier game and the fans will rightly expect your new game to be "more of the same". Because that's what the developer is TELLING them they will get by putting that title on there.
Short story: Developers want to have their cake and eat it too.
I am one of the people who thought DA2 was a betrayal because Bioware marketed DA:O as an RPG (they marketed it so hard they based a pen and paper game on it) and DA2 seemed to have most of the RPG elements pulled out of it.
I keep seeing this assertion made, but it seems like bullshit to me. DA2 took away your ability to dress your companions however you wanted to. You can still play Barbie Dolls with your main character, just not your companions. You still get to level them up and equip their weapons and accessories, just not their clothes. How does this become "most of the RPG elements?"
There is no doubt that DA2 had its flaws, like there only being one cave map that was re-used throughout the game. It shipped with some bugs which have still, to-date, not been fixed. The story focuses more on what happens to Hawke than on what Hawke actually does. I'm not suggesting that no criticism of DA2 is appropriate, but I do think that the assertion that BioWare removed "most of the RPG elements" is pretty vapid.
Edit: and it is worth noting that you can modify your companions' appearance, to some degree, through story choices. That's pretty solid RPG stuff right there.
Change is good if it improves and fixes crap from the previous game. Although too much change can make the game a different experience to the original. It still has to keep the feeling and tone of the original.
I'm not a big fan of too many true sequels in a game series. But a couple that are mentioned are in my opinion worth ridicule and criticism.
Metal Gear Solid 2 (MGS 2) was bashed for the use of Raiden instead of Snake for the majority of the game. But I played through it and the biggest problems I had with it were the overly long cutscenes and lack of significant amount of gameplay. Part 1 had variety. MGS 2 had a lot of running around doing the same thing over and over again. The only problem with Raiden was his overly annoying relationship woes. I hated hearing about it and I didn't feel that I could identify with him at all because I usually thought he was just a jerk.
Dragon Age II has a more significant flaw. The gameplay sucks. Whether this was considered dumbed down or not, I had no objections to the new story or characters. BioWare does a great job with those. One problem I had with the gamplay was their choice to move the series to a hack and slash RPG style. I am fine with that choice but the fact that you create a hack and slash RPG without the use of a block button just seems like a huge rookie mistake. I felt like I was running around hitting the enemy and then just running away again.
I don't think sequels are bad. And I don't think that innovation is bad in a series, but there has to be a balance of what made the series popular with what they are trying to do.
And yeah, people are afraid of change. It sucks, but that's the way it is.
No. People didn't hate Dragon Age 2 because it had 'change', people hated it because it was a sloppy cash-in on a much, much better game.
Further, not all change is good. Dislike of 'specific' changes in a game is not synonymous with disliking all change.