Awesome, but Impractical: America, you're my country and I love you, but why are you hosting a big ass party with a giant stage and projector screen when we're recovering from a recession?
Probably pushed all the expenses onto another, more financially advantaged, country like he did to Japan for a Christmas party in the anime. Other options are AU, Rule of Funny or Rule Of Cool.
Badass: Gladiator!Germany, hanger!Belarus, pirate!Denmark, hunter!Spain and shotgun!Grandmano.
Dragged Into Drag: Hungary takes way too much pleasure in putting Austria in a dress (he wore the costume under the condition that he wears pants while dressed in it.)
Chekhov's Gun: England writing on America's twitter. Yes, that wasn't just a throwaway joke!.
Cliff Hanger: Someone shows up when the party is over. We just don't get to see who.
Costume Porn: DUH. Sherlock!England, Francer Pan, Captain Cook!Seychelles, Tinkerbell!Monaco, Toy Story!Micronations, pirate!Nordics and Wonderland!Soviets to name a few.
Flipping the Bird: Belarus to Denmark. Mochimerica to Russia. Romano in the text message for Luxembourg.
Foreshadowing: The Batman logo on America's suit. Guess what his costume turns out to be? Funnily, in spite of furious speculation about what America's costume would be, almost no one noticed this!
And he spontaneously grew it back. Yes, England was indeed confused.
Product Placement: Belgium as Red Riding Hood mentions that a video game based on one of her horror stories called The Path is on sale. Also, Iggy made a sarcastic comment on America's twitter, and was his first follower. Also, the prize for the costume contest that England doesn't know about is Uncharted 3.
They Clean Up Nicely: Pretty much everyone does this. Of special mention are: Taiwan as the virtual idol Silverlight, Wy in a cute doll-like dress, Hungary as a handsome Prince, the Slavic/Baltics as an Alice in Wonderland ensemble, the Nordics as pirates, and India and Prussia in suits.