Honestly... Lots of people can have different opinions on different characters but it doesn't end up meaning anything. If you want to say that you think a character will be in brawl, give reasons.... and if you don't think a certain guy will make it, give reasons as well. How obscure or well known a character is kind of debatable, but how important a character is to a series or the fanbase can be better defined.
I'll give my stance on Petey Piranha...
Petey Piranha is somewhat important to the Mario series. He doesn't exactly rank as high as Yoshi, but certainly higher than King Bob-omb. I think this is mainly because he is a much newer character. He has appeared in many games and has become pretty recognizable to the Nintendo fans, becoming one of the recurring characters in the Mario Sports games, usually being one of the last unlocked. He is a boss in many of the handheld games and of course in his first appearance, Super Mario Sunshine. He has not been very well developed and it is mostly a mystery as to what his motives are and what exactly he is. He can fly, create whirlwinds, shoot gunk, and spawn piranha plants in various ways.
I don't think many care if Petey makes it or not. However some Mario fans, usually older ones, dislike him and don't think he should or will be in brawl. Honestly though, Petey has plenty going for him: For one, he has appeared in a cutscene. That alone is more than most would-be discussion topics can say. He also can perform aerially (he flies), and is a villain. All of that combined and you have a character that seems pretty likely.
Still, in the cutscene he appears in, he seems much too large to be playable. Not to mention that Petey doesn't exactly have a big or even existent fanbase, at least not in America. For all I know he could be a very popular Mario character in Japan.
With all of this in mind, and the knowledge that Sakurai doesn't care how new a character is, is willing to adjust a character's size, can give a moveset to anything, and has even given Delfino Plaza as one of the levels, there isn't really a reason to say that Petey is unlikely.
There. I've already said most of this, but this time it's all in one post!
Edit:
CasMat, I've already said Petey wasn't even ON Sakurai's poll.
Oh... didn't notice I guess *rereads thread*