In my Blockbuster spending spree I picked up three different xBox 360 games all of which are sequels to games I loved. What is my Final Fantasy marathon if not rolling around in sequelness? If you look at the top selling games it's pretty much sequel after sequel. (360 - Halo, Call of Duty, Gears of War, Grand Theft Auto, Fable PS3 - Gran Turismo, Call of Duty, Uncharted, Metal Gear Solid, Final Fantasy Wii - Mario Kart, Super Mario Bros, Smash Bros, Mario Party) The Wii has a few new entries on the list (including the original Wii Sports if you count bundled games) but they all also have sequels up there as well. Sequels are a big market. But what do people want from a sequel?
Personally I've been trying to figure out what I want from a sequel to try to quantify how good the new games I got are objectively before I play all of them. It's difficult, because I think about previous sequels that I've really enjoyed and the ways they evolved has been very different. Is it even fair to call Final Fantasy VI a sequel to Final Fantasy V or Final Fantasy IV? If not is it actually fair to call Final Fantasy X-2 a sequel to Final Fantasy X? A couple characters carried over between games and it's set in about the same world but from a gameplay point of view they're drastically more different than FFVI and FFIV are.
When I was a teenager I used to buy every version of EA's NHL series. The games didn't really change much. You get new rosters and maybe a new feature each year but pretty much you're just playing hockey. Shouldn't I have been content with playing NHLPA 93? Did I really need NHL 94, NHL 95, and NHL 96? Maybe not, but if I enjoy a game in the first place then a couple minor tweaks to shake things up a little and a new coat of paint in terms of updated rosters is great. It's a little new and refreshing but it's still the same game at the core. This is the safe way to do a sequel and the Madden franchise in particular has made an absolute fortune pulling it off.
On the other hand you have games that use the same name but go off in a completely different direction. Super Mario Bros 2, for example. (This actually was a different game entirely that just had the Mario theme painted on top near the end of development.) It has some familiar characters and features but plays like a completely different game. Most of the Final Fantasy games fall into this category as well. You may choose between attack, magic, and item. You may cast Lit, summon Shiva, and ride a chocobo. There's a dude named Cid and he just wants to fly. Possibly there are crystals and fiends corresponding to the 4 elements. But the story lines, the combat systems, and the way you level up change drastically from game to game.
So what do I actually want from a sequel? I still don't know. Mostly I want a good game that I enjoy playing. An easy way to pull that off is to basically release the same game with a couple tweaks. This can be seen as a cash grab by some people especially if the game doesn't change much but I'm pretty ok with it. Going with a completely different game with some similarities certainly can work but it still needs to stand on its own as a good game. Super Mario Bros 2 certainly got some people to buy it because it had a Mario theme but if it wasn't actually a good game at its core it wouldn't have made it.
I feel like I want Dead Rising 2 and Portal 2 to basically be the same games as their predecessors. Those games had an interesting hook at their core and I just want more of the same. All I want is more ways to kill zombies and more puzzles to solve with spatial reasoning. Overlord 2 I actually want more out of. A completely different but good game with the same basic theme. I'm evil! Run with it!