That's a bit of a bummer. I was never an IC, Lucas, or Snake main in neither Melee, Brawl, or Project M (Melee referring to IC), so I guess I don't feel as sad as those who are dedicated players of those characters. Still, it's going to feel pretty weird without at least ICs, since they were a mainstay since Melee.
Snake, I can understand. I'm not too surprised by his exclusion, though the inclusion of Sonic from Brawl did realistically make Snake's return a possibility.
Lucas... well, I guess the Mother series has died down in profile, and nowadays, the vast majority of the worldwide audience thinks of Mother 2/Earthbound when they think of the Mother series, and given Ness was one of the original 12 from Smash 64, it made sense that he'd stay. Lucas, on the other hand, was from a more obscure title in the same vein as Ninten from the original Mother, and so it could be argued that Sakurai thought Lucas wasn't as significant to put into the game as other Brawl characters, like Meta Knight, King Dedede,etc.; characters nearly everyone knows. I mean, I'll be honest, I don't know much about Lucas outside of his description in his Brawl trophy. Combine that with a lack of official localization, and one can understand why Lucas was cut. No one has to like it, but they should at least understand it.
Wolf is a bit of a bummer too, but maybe Sakurai didn't want too many "clone" characters, which wouldn't make sense given Dr. Mario, Lucina, and Dark Pit (assuming he is a clone) are purely clones, while Wolf is more of a "Luigification" of Fox and Falco. Hell, Falco is more a Fox clone than Wolf. I don't understand his exclusion either.
Ice Climbers, well, I couldn't begin to tell you why they'd cut them out. I could say it's because it'd be too complicated for Smash 4's engine and what they were trying to aim for when it came to balance and simplicity. It could also be argued that IC's chaingrabs were seen as too OP and they decided to cut them for that as well. That first argument, however, is essentially debunked with the introduction of Rosalina, who is somewhat similar to ICs in the sense that you're controlling two characters. Honestly, no one can know for sure until something has been confirmed.
Mind you, this is assuming the leaked pics and videos are indeed the real thing.
tl;dr: R.I.P. ICs, Snake, Wolf, and Lucas.