Coat too tight and too short, trousers too short and too tight through the thigh. Single vent looks awful as well.
Burning may be a tad too harsh as long as you can get your money back.
Bah don't worry too much. You said it yourself, you were going to a slim looking suit and it's only for fun. The suit is fine. Yeah, it's a little tight, but so what? Tight suits are "in" right now. Besides you probably don't want to go around looking like you are ready for a board meeting anyway. I'm sure someone will rage against this as hard as they raged against your suit, but honestly? If this thing is only for fun and whatnot, I don't see the issue. A full trouser with enough fabric to drape all the way to the shoe is out and looks old man-ish.
And also: the jacket is too short? Please. His jacket covers 95% of his bum. That's totally acceptable. Short jackets are also in right now.
That said, what everyone said is more or less spot on. There's a lot of pulling on your back vent and the pull around the button coupled with the bowing of the lapel means the jacket's too tight. So maybe wear it unbuttoned or something. Pants look fine, if slightly long for current fashionable taste and the jacket sleeves are a hair too long. If the sleeve buttons aren't functional, I'd suggest you go take it into a local alteration person and have them raise the sleeves by 1/4 an inch-ish. Ideally you'd like about 1/4-1/2 an inch of cuff showing.
Keep the jacket, burn nothing, wear it to parties.
1) Lurk longer.
2) Come back and read above comment.