A thought occurred to me recently regarding the lack of unlockable content in games. Maybe I'm just not looking hard enough, or not reading any GameFAQs, but many games have no readily available hints that there's any sort of unlockable content. Take the newly released "The Legend of Zelda: Ocarina of Time 3D" for the Nintendo 3DS. There's no clear indication that there's any new content in the game aside from the original port, but it does have a "Master Quest" port (a harder, altered version of the game) and a "Boss Rush" mode. Had I not looked this up, there'd be no indication there was anything there.
Another thing that always bothered me were unlockable items and content that don't mean anything beyond being merely aesthetic. Like the impossibly huge number of hats you can unlock in Team Fortress 2. They don't mean anything. They're nothing special. And you get them for stupidly simple reasons, though no doubt a huge number of them involve spending money. There are countless deals daily on Steam for new, non-Valve products that offer hats to unlock in Team Fortress 2 should you buy these deals.
The hats may not be meaningless depending on your perspective and how much you value aesthetics, but the fact that you unlock them through buying whatever game Valve is supporting on steam makes them meaningless.
Unlocked content should be related to the game. You should be able to access it through completing objectives or goals within the game itself rather than hunting down obscure codes or making out-of-game purchases to get them. This adds meaning to whatever you are unlocking, because you are earning it. A silly concept in this day and age, I know, but it's an important one. Human beings feel the greatest sense of accomplishment and reward when they have earned something, not when it's handed to them on a silver platter they paid for or got for doing nothing. Does anyone care about Achievements in games like "Beat the game!" or "Level 85!"? No! People care about the Achievements that take time or are difficult to accomplish, like WoW's "What a long, strange trip its been" that involves completing every single holiday achievement in a year. Of course, it rewards a unique mount, but that right there is an unlockable item that has people clamouring for it every year.
The other thing games need is unlockable content that comes in more than just aesthetics or game play varieties. It needs a mix of both. A great example of this is the "Tales of" franchise of games, and I'll have to give a nod to "Tales of Vesperia" for this one. Not only do you have a number of side-quests, items, and other things you can unlock to get costumes to change the appearance of the game characters (from serious to silly designs), you also have a number of new features to unlock that affect gameplay as well. While this is more prevalent in the PS3 release of the game (Japan-only, currently), its still there. There's a secret dungeon hidden after completing the game. There's several types of arena modes. You can unlock a second tier of Mystic Artes (think like an "Ultimate" or "Overdrive"). Side-quests and other story lines that cannot be explored until the game has been completed, etc...
In other words, the unlocked stuff in Tales of Vesperia easily makes up for a good chunk of gameplay. The main quest is but one aspect of the game, and alone it can take 30-40 hours to complete. Unlocked content can easily add nearly 50% more game time if you explore it all.
More games need to be like that, and not just JRPGs. Every game needs something in there for the player to strive for after the main storyline has been completed. It's disappointing that in this day and age, we have developers that focus only on getting a product out and don't focus on any form of replayability, then they wonder why their game ends up being resold in the $10 bin at a Gamestop.
Of course, my choice of unlockables would be different depending on which one of my projects I was talking about. In my MMOs, additional vanity items and rare drops / difficult, long quest chains would easily act as venues to unlock content. In a platformer, I'd have additional characters that have unique gameplay elements that make playing through the game again fun, and of course it would need "Boss Rush" and "Horde" style modes too. A MOBA would have players unlocking new champions as they are released, as well as new skins, by playing the game and earning some form of currency (a feature common in MOBAs), so that they can pick and choose their rewards as they come in. I think you get the picture.