I for one love unlocks, provided they are doable. Anyone remember the Krypt from Mortal Kombat Deadly Alliance? Sooooo much stuff to unlock. There were tons of stages, characters, artwork, concept art, trailers, etc. Really gives incentive to keep playing. However, they did it right in that you got tokens for pretty much everything you did, and could spend them on unlocks (ala MVC2's store).
I hate unlocks like "beat on impossible difficulty without losing a single round, perfecting at least half of them, to unlock a character"...