Where do you stand on unlockable content in games? Do you try to unlock everything or are they just bonus features of no interest to you?
For me, it's a love/hate thing. I like the concept, it gives you something to play for and they can serve as a good incentive to try out game modes and the like that perhaps might not check out otherwise, but poorly implemented they're incredibly frustrating. Unlockables can extend a game's longevity but ideally you should be able unlock them soon enough so you're not completely bored with the game by the time you've unlocked everything.
A good example of unlockables done poorly would be Smackdown: Shut Your Mouth on the PS2. Items are unlocked by winning matches at PPV events in the story mode which is fine, but you could only unlock one per event (selecting from a list of available items to unlock depending on the event) which meant you had to play through story mode umpteen times to unlock everything, which got repetitive and given the loading times on the PS2 meant if you actually managed to play through enough times to unlock everything, there's no way you were still fully into the game by that point. Smackdown vs RAW 2006 also had the cheek to have Jake Roberts only unlockable through owning the PSP version as well. I ended up unlocking what I hadn't already unlocked in those games with an Action Replay.
Conversely, Mortal Kombat: Armaggeddon had a pretty good system for its unlockables. You could either earn money to buy them from the Krypt or you could unlock most of the stuff that was of interest by playing through the Konquest (story) mode. That way, depending on your patience you could unlock the good stuff first and go for 100% completion in time. That's the way I think unlockables should be; not so easy that you've got them all within the first day of playing but unlocked quickly enough so you can take advantage of them while you're still into the game.