Are we talking about actual hype that i as a player fell for or just insane hype in general?
I'm not an "old school gamer", so this question doesn't really apply to me. That said, the first major disappointment I had in a game was Dragon Age 2.
I liked, no, LOVED the first Dragon Age. It was a masterpiece. I was excited to relive all of the feelings I enjoyed in the first one, only with exciting new sequences and characters, but... it just felt so unspectacular. Don't get me wrong. Dragon Age 2 is a good game. But it just wasn't great - at least, compared to Dragon Age: Origins. What was even more crushing was that Dragon Age 2 was released on my birthday. The one chance I'll probably ever have a game series I truly loved release a new installment on my birthday was a disappointment.
As the years have gone by and I've adjusted to the transition between Origins and Dragon Age 2, I've begun to appreciate the middle child of the trilogy (Because we now have Inquisition) more. I still love the characters (Isabella is the greatest piece of fiction in anything ever), I still love the story, but I'll always remember how disappointed I was on my birthday when the game was first released.
Spore - Peter Molyneux - Ubisoft- WoD.
Cod has a new campaign, new weapons, new multiplayer levels every year. Zelda has been recycling the same weapons, villains, and dungeons since the 80's. Zelda recycles enough to make cod blush. The same weapons, villains, dungeons, and princess in every single Zelda for the most part. It's almost as cheesy as bowser vs Mario round 35
Daikatana is the earliest I can remember. Everything else that springs to mind is what I consider "modern" games(ahem Diablo 3).
Spore hands down.
Not really old school...but part of a long-running series:
Metroid Other M.
Dammit Nintendo, you made me lose faith in you with that one. You licensed it out to Ninja Team and they turned Samus from a quasi-silent protagonist (as she had some dialogue in a couple games...but not much) into a whiney little stereotype anime chick...
Let's not forget the gameplay changes, the quasi 2D/3D was okay, but having to stand still to use missiles with the Wiimote was horrible, and they cut out nearly all of the exploration that the series was known for. In fact, the game felt more like a Wii version of Metroid Fusion....only not as good. Honestly, look at the game, the setting is almost identical to Fusion and it's even more Linear.
There is a thin line between not knowing and not caring, and I like to think that I walk that line every day.
I honestly can't remember any game I purchased that had me hyped up and failed to deliver, I either loved it or traded it in/got a refund, however I have seen a few games get so hyped up then fall flat on their proverbial faces upon release.
Daikatana on the PC and Fighting Force on the PSX/PS2, touted as the next gen Streets of Rage but it was garbage. Those two are the only ones I can think of (atm) that had many people excited and promptly left feeling fucked over.
That's the thing about Molyneux. He isn't bad when he has someone holding him on a leash when he was leading Bullfrog and EA was the voice of sanity within that Hierarchy (yes EA in the 80s and 90s were GOOD). But when left alone that is when things go really terrible fast. He just tries too much, promises the moon. Wants to deliver the Sun but he gets too ambitious and doesn't accept limitations.
Oblivion taught me to be more sceptical. definitely didn't live up to the hype.
I liked pretty much every game I played when I was young. Maybe I was just easy to please. Or maybe it was because I never got new games very often.
The only game I was super hyped for that was a big letdown for me was Elder Scrolls Online.
To some extent, Neverwinter Nights. It's strange to say that since after a while I did come to love it, but I got myself really hyped up for it while playing through Baldur's Gate only to find that the stuff I was excited for had changed completely by release. In particular, I remember the BG2 load screen messages hyping up how you could transfer your character to NWN, and that it would be a sequel to TOB- nope, it was either cut or completely false advertising. I get that it wouldn't really work with how NWN ended up (it's a low level campaign in a different edition ruleset), but it was still very disappointing. Couple that with other changes like how they had you control one character with a single companion rather than a full party, and I was very let down for a long time before I came to enjoy it.
More recently, Guild Wars 2. I mean, it's okay, and I still go back and try to get into it, but it didn't even come close to living up to the hype- or even its predecessor, GW1. It's not like the game's actually missing anything they promised, but they failed completely at having its features accomplish what they wanted to do in the manifesto? Events to create a dynamic world? Nope, it's just the same few repeating so you see everything over and over. No quests so you'll experience the dynamic world instead? Nope, you'll spend most of your time doing hearts while levelling, which are even more generic quests. Cosmetic only armor to end grinds? Well, sure, you don't have to grind now, except everything in the game turned out grindy so they just made it all optional (I don't know about raids, super world bosses, or high level Fractals as I haven't gotten into the game long enough to do them yet, but those were all post-launch anyway). Scrapping the trinity to give you further freedom to customize your character? Nope, zerker or you're useless since there's no need for anything but pure damage (with builds that automatically send out boons). I really should go back and see if the newer high end stuff fixed some of this, but so far they've failed completely at pulling off their manifesto.
The really sad thing is, outside of the dynamic world stuff, they already did an amazing job at doing all of this in Guild Wars 1. If they had just continued expanding on it, or at least used it as bigger inspiration for loot, customization, and challenges, I'd still be a huge fan of the series. Instead, GW1 no longer gets updates since they moved onto it's disappointing sequel.
Sonic Heroes and much later, Guild Wars 2.
Sonic Heroes had the privileged (or curse) of following the Sonic Adventure games and, well, it fell flat for me. At the very least, they could've kept the Chao Garden. There's a feature that we'll probably never see again.