Let's take a look:
Wii Fit Trainer - Nintendo kept banging on about the Wii Fit, and many people do like to use the Wii Fit. It's either Wii Fit Trainer, or the Wii Fit board. Wii Fit Trainer practicly had a moveset all set for her already, and the Wii Fit Board is basicly Assist Trophy material anyways. So I'm not too surprised about this. I just love the troll factor her character has.
Mii Fighters - Similar to the Wii Fit Trainer in the fact that Nintendo keeps banging on about them. They also have a bunch of fun games that many people love (even if the hardcore crowd can be snobbish and dismisses them). Mii Fighters also let's everyone put whoever they want into the game, and have more customization which is always a bonus. Surprised they had 3 types of Mii Fighters, but I'm glad they did, since the 3 types they chose are the common methods for characters to attack in anything.
Villager - Animal Crossing is one of the more popular Nintendo titles, so not too much of a surprise here. I haven't played Animal Crossing yet though.
Megaman - Once I heard that they will have a Capcom character, my mind went immediately to Megaman. When you think of the other popular Capcom characters, then look at Megaman, he has no competition since Megaman started out on the NES, and helped the video game industry a ton.
Pac Man - Not sure if it's the case anymore, but when I was younger, even if you've never played a video game in your life, you knew who Pac Man was. He was practicly one of the main icons of video games. He had to be in. Especially with the knowledge of the company that is helping make Pac Man.
Palutena - Kid Icarus has been getting rather popular, and it is something Sakurai is working on so not too much of a surprise here. I was only a little surprised because I wasn't sure if the new Kid Icarus was popular enough to get Palutena in the game. Even if it wasn't, it still makes it so people are even more aware of the new Kid Icarus. Besides, she's a fun trolly character.
Greninja - Off and on I would consider a X+Y pokemon, just because of how drasticly different it is compared to previous pokemon games. Also, consider that X+Y is what drew me and many other people back into the Pokemon games, and it shouldn't really be a surprise that a X+Y pokemon came in. Greninja's only real competition was Fennekin, as I never hear people talk about anything else when it comes to starters. There weren't any other pokemon that had huge star power, some got close though, like Hawlucha (though personally I don't see why people love hawlucha so much). So the fact that Greninja got in shouldnt be a surprise.
Punch Out - He was incredibly popular back in the day, it was not uncommon to hear people talking to each other about it from time to time. I'm not sure how popular his new games are but they seem to be decently popular. I'm not sure what other contenders there are for a new game series in smash are, but Punch Out definently deserved Little Mac. Though personally I loved the game, not Little Mac himself, but whatevs. He can be the cool underdog like Rocky or something.
Rosalina - Out of the whole list, Rosalina leaves me scratching my head more than anything. Sort of how Ice Climbers left my head scratching in Melee. The Ice Climbers never seemed popular, at least in the USA, as I felt like I was the only person in the world who played the game. The only major thing going for Rosalina I can think of, is that Nintendo wants to show that the Mario games really are changing and aren't the same thing all the time. However, Nintendo seems to be shoehorning her into many other Mario games now. Which makes me wonder if she's going to stick around for other major Mario titles, or if she'll only be a huge deal in the galaxy games. I am really surprised that she was added instead of Bowser Jr. as Bowser Jr. has been somewhat important, and he also represents Bowser's past fights, and the Koopalings as he's practicly a half-leader to them, while being his own special thing while representing that Nintendo has done some newer things.
I think Nintendo is toying around with Toad's identity right now (with Blue Toad and Captain Toad), so I'm not too surprised he's not in now.
Even the characters I think deserve to be in the most should not have been put in above the others. Sakurai just keeps surprising me with characters I don't usually think of.
Ridley - A.K.A. the Meta Knight for Samus, but also has story behind his rivalry.
King K Rool - I dislike K Rool and the Donkey Kong franchise but people talk about him a bunch (more than anything else in the DK games) and he was the final boss for multiple games, sadly DK's newer games seems to want to forget K Rool which does hurt his chances quite a lot. There's kremlings in Smash Run though, we'll just have to see.
Mewtwo - Personally I'm on the fence whether or not Mewtwo should be in over Greninja, but really they should be in for two different reasons and it's hard to compare. Being the focus of the first movie, and being the first big bad legendary, along with his title of being the "strongest" pokemon, along with being the final boss of several spin off games, he definently deserves a spot in the roster.
Remember, even if the character feels like it will have a boring gameplay style, a little imagination can go a long way to make that character fun. Don't judge a book by it's cover.

Example: While I figured Miis would be added, especially after hearing the customization announcement, I still figured they'd be boring. It looks like they are going to be one of the most interesting characters to play now.