There are lots of different ways "reveals" were highly anticipated.
Assassins Creed 1, was supposed to be the most insane game changer ever. People saw the demos. Every reveal it just looked more incredible. In 2006 nobody had seen animations like that. It came at the peak of the parkour craze, there was an intense desire for a new IP like that. I mean, everything about it. Every single thing was supposed to have made it Grand Theft Auto in the holy lands.
The game ended up being not bad. Not bad at all, but disappointing. Limited shallow combat, a story with promise that was just sort of okay-ish, a premise that was bizarre and generally disliked (future / animus stuff), repetitive gameplay (lack of mission variety).
Brute Force, was a new IP for the first Xbox that looked really solid. A dynamic hot swap-in swap-out 4 player co-op game it was designed to be the first next big thing post-Halo. But when it landed in 2002 on massive hype it turned out to be disappointing. The art in that game is still to this day some of the worst.
Splinter Cell, was the next big thing that came out for Xbox after the amazing KOTOR, and it was quite expected. This was the first time the general press and gaming public understood things like dynamic shadows. It was the big selling point. Amazing atmosphere, a true stealth game like never before.
Turned out to have a boring story, and trial-and-error missions that has haunted the series ever since. The 8.7 meltdown from Gamespot is still massively legendary to this day.
In a post World of Warcraft world, many MMORPGs got an early or premature death as they were riding on massive hype into mediocrity and disappointing. Before WoW in 2004, the small but passionate MMORPG community only really sought to expand the MMO genre. Nobody really expected a game like WoW to come along and completely change a genre like it did. As WoW kept growing, it has to have been the long wait for both Warhammer Online and Age of Conan that stung the most. For both of these massive titles to fail to outlandishly. It was a dissappointment without equal.
Spore, in 2006 is another game for PC that was supposed to have changed everything. Not since Project Ego ("Fable") had I personally seen a gaming community taking so much out of context turning hype into factual statements about lies. Spore was not a bad game but just overly ambitious and it failed on that.
Hellgate London in 2007, have to be my personal one. Goddammit this one hurt. This one really really sucked. Bundled with a terrible subscription model they announced shortly before release, the level design totally sucked. Endless randomly generated london sewers and post apocalyptic nonsense, HGL was a major major disappointment. What it did amazingly with art, loot, and inventory it ruined with everything else. This was supposed t have been the next Blizzard. This was the real spiritual Diablo 3. What a mess this ended up being.
Rome 2 Total War, no Total War fan expect a new Total War game to be bug free, but nobody expected the sequel to the most significant Total War, to have been so disappointing. Particularly not after the highlight that was Shogun 2.
Rome 2 is for all intent and purposes 1 step forward in graphics, but several massive steps back in everything else. Not a great game by any stretch of the imagination. Much was lost here. Political system, removal of family free, a lot of the identity and careful attention to detail that made everything feel alive was replaced by a grotesque lack of quality that made you unable to care.