...I seriously don't get it.
What the hell do you people possibly see in this guy? O_o
What's with all the hype? I just don't see it.
A lot of people love his game.
A lot, I mean A LOT of people like the determined, hard working everyman type of character(complete with training montages!) which Little Mac represents. It fits in with the idea of the American dream AND the stereotypical hero archtype that is incredibly popular in Japanese media like Manga.
Despite being basically mute(like a lot of Nintendo character anyway) and being from a game with essentially no plot he's actually got a lot of personality in his expressions and such that have made people attached to him as a character.
A lot of people like when a series has a character that's basically a regular guy that can fight toe to toe with a bunch of super powered freaks (like Batman, the Punisher, the one "badass normal" in most animes). Especially when they get to play as that normal guy. Little Mac certainly qualifies as that in Smash Bros. and he did in punch out to, since most of the cast are basically crazed circus freaks(no offense to anyone who actually works in the circus
).
He also strikes a cord with a lot of people who feel they can't succeed because they have this problem or that due to his small stature. He's a role model type character, basically. I've heard multiple people point to the cinematic where unflinchingly he sizes up against the monstrous Mr. Dream in Punch Out Wii as an unforgettable, heart tugging moment for them.
Boxing characters have been a staple of fighting games since forever and tend to have a very distinctive play style that a lot of people are very fond of and prefer to play as now we finally have one in Smash and he seems to fit the boxer character type very well. Also he looks to have a play style that competitive Smash players are going to salivate over.
He has a unique gameplay gimmick that a lot of people were hoping would show up in Smash in some form(a super bar).
People were wanting unrepresented serials to get representation and Punch Out is one of the most famous/successful to still not have a rep.
He had a really fleshed out assist trophy in Brawl that made people imagine him as a playable character and the Punch out stage reveal in the E3 trailer got people really hyped for his possible reveal.
He got a really cool reveal trailer.
Doc Louis seems to be somehow involved in his character at least going by the trailer and he's very popular for his "spunky black man" attitude that a lot of people find amusing not to mention he has meme status.
I could probably think of several more reasons if I thought on it longer.