I agree with Arnell..
Unlocking stuff always gave this nice sense of achievement (no not achievement unlocked like in xbox). I like being rewarded for my efforts with costumes, videos, art galleries. It's sad that not many games grant this these days.
I remember in SFA1 when you beat the game on lvl 8 without losing a continue and beating Akuma. that was the only way to get him unlocked forever without having to put in the code. It was a pain to do, but it was also an awesome reward for doing it. Same for Bison and Dan.
DOA3.1 had some insane sets of rules for the extra costumes... God those XBOXes in survival would never come out. but once you got them, again, epic!
People don't seem to like unlocking characters, personally I dont mind it. I find it quite enjoyable acctually, especially in a game like DOA. Its kinda easy to go through everyone since all the characters share core elements. Its not as drastic as say switching from a charge character, to a grappler with 360 moves, to a fireball motion character in SF. In DOA the reward wasnt just the characters, it was character + cinematic + costume... which was pretty sweet.
I kinda like how MK did it you get cyber subzero from finishing story mode and, with cash prizes and the tower you unlocked costumes, concept art, music etc. It was awesome beating the tower n getting that last costume

, even though I never used it lol
I hope they keep that element in DOA5. As much as I dont care about Ein, I would be really happy if somehow he secretly made the cut as an unlockable. I dont see why he wouldnt either, hes pretty much a name + move swap more or less..