It's a good point. Especially if you put some number on the end of a title, as in, a direct sequel, people expect generally the same gameplay. The problem is that you'll never, EVER find the balance between too similar and too different.
Dragon Age 2 being one of the extremes of too different and something like the recent CoD games being the extreme of too similar.
Even then you still have people saying that one or the other of those is better than the original. When you hit the extreme you'll tend to have maybe a half and half split. In an optimal sequel you'll probably have around a fourth saying it's too much the same, a fourth too different, and half saying it's better than the original.
Dragon Age 2 being one of the extremes of too different and something like the recent CoD games being the extreme of too similar.
Even then you still have people saying that one or the other of those is better than the original. When you hit the extreme you'll tend to have maybe a half and half split. In an optimal sequel you'll probably have around a fourth saying it's too much the same, a fourth too different, and half saying it's better than the original.