I am generally against sequels, Pikmin should have been a one off, in my opinion, as well as many other games, such as Mario Galaxy and Portal.
Of course there are games that I'm glad had sequels, I'm happy that mario and zelda weren't one-offs.
However, sequels encourage lazy and unoriginal developers, the Call Of Duty franchise has had little / nothing new since the 4th game, L4D2 should have been L4D1 DLC.
Which brings me to my next point, DLC. DLC is a great way for developers to make money without awful, samey sequels. Of course some developers (*CoughActivision*) do both! I just find that too greedy, not to mention the outrageous DLC prices.
Now let's look at Minecraft, a great example. I paid £10 and I get every update free, they continue to make money due to their game being so good and having a great price.