It's actually the opposite. It's optimism. People would never expect Sakurai to NOT include Mewtwo and Ridley, right? ...oh, he actually didn't. Well, surely Smash Run is online, it's such a big mode, after al--it's not. We sure are looking forward to another nice 25+ all new stages, aren't we? ...half of them are re-used or also in the Wii U version.
...sigh, at least it's a definitly a completely fresh game, right? Many voice and soundclips are re-used, it takes only Brawl's techs and removes even more of them, and 70% of the soundtrack is reused music or that damn main theme.
People had standards for Smash 4, acted like it was going to have so much cool new stuff in it. So much speculation, thrown into the pits for joke characters, no online, and lazy recycling of content. Is Smash 4 automaticly a bad game because it didn't live up to it's speculation? No, people just expected TOO much. They were optimistic.
Lots of games die because of this. It's a vicious cycle, really, where people hype up games so much that after playing them, they become disappointed quickly and start lashing out. Childish, really.
Final Fantasy XIII suffered because of this. People expected this giant, awesome game when in reality, it was more like Final Fantasy X where it was linear until mid-late game. Ironically, Final Fantasy XII was less linear - you can go anywhere, but don't expect to live - and people bashed it for that, its combat system where you prepare for fights, etc. Final Fantasy XV and Kingdom Hearts III, to me, will probably suffer this as well. Mostly FFXV which is to many this "saving grace", "redemption", or even "the best thing since Final Fantasy VII". You will be sorely disappointed if you do that.
I think Battlefield 4 had a little hype-overload, Watch_Dogs even with its performance issues isn't that terrible, and the Operation Rainfall trio suffered as well. The Last Story was said to be decent at best, some didn't like Xenoblade: Chronicles at all, and Pandora's Tower played more like Zelda when people probably expected something else.
Then there's the opposite where hatred and negativity prevents the game from being appreciated at all. Case in point: DmC: Devil May Cry. All the blame went towards Ninja Theory when Capcom specifically asked for Ninja Theory to change Dante as drastic as possible. They even went for NT for story purposes and honestly, DmC's story is coherent, simple, and not a chaotic mess like the original Devil May Cry games. Then again, Capcom ignored plot so much that DMC ended up really messed up. Combat looks like its more like DMC1 and DMC4 combined. Nothing special, but nothing horrible like people make it out to be.
Don't get started on Pokémon. Everyone expects that Pokémon Red & Blue experience. That's your first Pokémon game, of course it's amazing. Nothing can replicate that. I messed around with Emerald, but nothing can replicate my first, legit journey in Pokémon X; my first Pokémon game. I will never expect other Pokémon games to be like that. It's illogical and hopeless. Will I expect a fun Pokémon game? Hell yeah. I won't expect a perfect game nor the best game since "best" doesn't exist when there's always a better game waiting to be made.
So far, Metal Gear Solid V has tried to show little as possible and leave everything to players experience. Is there hype? Yes, but core stuff like gameplay is shown and it's what a Metal Gear game or a stealth game would play out as, but with everything combined so you can manage Mother Base, do side missions, explore, etc. Nothing special, but nothing short of amazing.
Contrast that with the little tidbits and teases Square Enix has shown with Final Fantasy XV then Versus XIII. Man, people are really getting hyped up it's not funny because of the inevitable outcome of crushed dreams. I really, really hope SE does a stage demo, explains the core features, and just continue along because nobody wants another Final Fantasy XIII incident. If they show it as "this is what's the game's all about" and not the cinematic, "Holy crap!" stuff they've shown again, then FFXV might not get as much backlash from post-hype.
That's not to say it's all the developer's fault. It's our fault to for over-expecting things. Just enjoy the game for what it is. Have fun instead of expecting fun.