Well, for me, just like Impeccably-Stressed said, the characters are of great importance. If they were somehow lacking in the first book, then it is unlikely I would ever consider buying the sequel.
Guenevere is a super fantastic example of how characters are the drive of a game. Arthur, Lancelot and Morgana are for now the main cast of people the player can interact with (there will come more though) and it is intriguing to see how each of them reacts to the choices Guenevere makes and how they interact with one another. It is amazing to see the extent of how Guenevere’s choices can actually make a difference.
Other than the three-year skip between each book (which is used as a ways to improve Guenevere’s skillset), there is a continuation that marvels me. It is written as such that I, though I may be alone in this opinion, do not have the feeling that there has passed that much time at all.
And skipping a fast amount of years can be bothersome for some sequels, which is why I bring it up. Because often I feel like I missed a lot of my characters life, especially when years are ‘taken away’. Mind you, it has never been a dealbreaker all on its own, but it does carry some weight in the decision to buy or not to buy… that is the question.
Furthermore, the setting of the game is of great importance to me as well. Par example: if I had to choose between a game with a Medival setting or a game with a contemporary one, I would go for the first. Though this is different for each individual, of course.
A good plot is always preferable, naturally, but not necessary for me. If you give me a cliché ending I am perfectly happy to buy a sequel, as long as the characters are interesting and have gone through at least a bit of character growth.