Crafting systems. They sound cool, and some games have done them well, but most of the time they just make sure that I never want to sell anything so I end up carrying around a bajillion random crafting components long after the only things they can be used to make have stopped being useful to me.
Sometimes I can make outdated things with components and sell the crafted items at a slight profit, and that can work well. But the worst use of a crafting system that I can think of off the top of my head? Dust: An Elysian Tail.
Otherwise, it's a spectacular game, but to craft anything, you need both a buttload of components and a blueprint for the item. Almost every component and about 80% of the blueprints are random enemy drops, which means lots of grinding if you need a particular component (or you can spend a ton of money on a couple of it at the shop, and then wait for it to restock so you can buy more of it). Most of the time, what you can craft isn't actually any better than what you already have or can buy. To craft things, you need to trek all the way to the blacksmith (unless you bothered to find the item that lets you contact her from anywhere). And while actually crafting the thing costs far less than just buying the weapon/armor/whatever from the shop, if you try to sell your crafted doohickey, you'll end up breaking even (which actually means you're losing money because you're also down the crafting materials). "So just don't craft anything if it's not explicitly better than what you have!" I hear you say. And you'd be right. Except unused blueprints continue to sit there, mixed in with your real items, just taking up space and cluttering up my inventory. And you can't toss blueprints out, even if they're for an item from the very beginning of the game and you're right before the final boss. This last part is probably just a nitpick, but it seriously bugs me; I like to keep my inventory screens clean, thanks.
My favorite crafting system on the other hand, is in the Etrian Odyssey series, because it's not really a crafting system. Instead, enemies drop random crap that you can sell to the shop, and if you sell enough of the right kinds of items, the shop will add new stock. This actually makes more sense to me than the usual crafting system, because instead of "Bring me this stuff and for a fee, I'll make something out of them," it's "Bring me stuff, and if I can use it to make something cool, I'll sell that to you!"
EDIT: Oh no, wait, Dust's crafting system is tied with the cooking menu in Muramasa: The Demon Blade for worst. In Muramasa, you find a bunch of cooking ingredients, and you can make healing items out of them. But first you have to buy the cookbook to learn the recipe. Why can't I just find the healing items already cooked and ready to use? All that this does is add an unnecessary step to things.