The worst part for me would be how it feels like so many things have been revealed without being left as a surprise. Of course, this could be proven wrong and I will not see until the game arrives, but by now we have seen many characters, items, and a lot of the Smash Run map. I guess there are still stages, music, and game modes. Though we all know the big one that would be desired to see as a surprise is characters when you unlock them. Other game modes not being revealed likely means that there may not be anything particularly special or new. Stages and music might be safe at least.
The game will be cool and fun regardless, but not having those surprises and secrets would take something special away for me. Revealing too much is likely the worst part about it for me. I know they want to show off the characters to try to lure players in, but the surprises and secrets are what I really like. I also feel like there has been a lot of this sort of thing anymore, lots of pokemon and mega-evolutions were revealed before X&Y's release which left only a few unrevealed, Super Mario 3D World spoiled that Rosalina would be playable, and Mario Kart 8 showed its entire roster before release. I would like to just avoid the information, but somehow it always seems to get posted somewhere anyway. I will at least try to stay off the internet entirely after the release in Japan. That much I can at least try.