I'd say it depends.
I think if you pre-order a game, find out it's shit before you get it, and want to put the pre-order money to something else then that should be fine (maybe this is already done, since I'm pretty sure the EB Games I go to offers that option)
Full refund after you get the game though? I'm not sure. If you were to do that then there should be an obvious timelimit, maybe like you have a week and then otherwise you only get the store-credit for trade-ins, but even that's sketchy. If the game is clearly broken (either the disc is damaged on purchase or the data is corrupted from purchase) then obviously that should be allowed, and I'm pretty sure that's offered even for non-pre-orders. However, if the game just isn't up to your standards (or anyone's really) it's not really the job of the developer/publisher to refund the game. Technically, they held up their portion of the transaction by providing a product that is at least "functional", as in the media it came on functions like any other, then it's more of how you feel about the final product more than anything else. While getting spoiled, rotten, or under/overcooked food at a restaurant will get you a refund, eating the meal and then saying it wasn't good won't necessarily get you a full refund. Perhaps some improved service, a small discount, or maybe something for free, but rarely do you get a full refund, because they technically provided a "functional" meal.
There is an exception though, and one I think could apply to Aliens: Colonial Marines. Even if the disc isn't physically broken, if it has a major bug on it (such as the save-wipe glitch) that actively prevents you from playing or completing the game then and is NOT exclusive to the disc but rather an aspect of the game, then a full refund should be considered fair.