The biggest let down i've had in terms of sequels is by far the Modern warfare franchise, after CoD4 being this fresh new sort of fps multiplayer game that was so much endless fun (although it did have it's fair share of problems) came the enormous amounts of hype for MW2 if anyone remembers, people thought that it would take the original Modern warfare and fancy up the graphics engine, fix the problems in CoD4 and pretty much try to 'perfect' CoD4(which would have been the best game ever). But it didn't, what it actually did was create a buttstuff of problems which were some of the most infuriating things i've had to deal with ever: OMA dangerclose noobtubes, Commando lunge, painkiller, final stand, Snipershotgun and machine pistol secondarys, removed juggernaut giving stoppingpower a massive power imbalance, the carepackage and javelin glitch which they didn't fix for like 4 months, nuke boosting and however many problems i left out. It could have been one of the best games ever so easily imo if IW took the time to just put out a few patches to fix MASSIVE problems which is one of the most frustrating parts... anyway i could ramble forever about the horrors of MW2.
And i'm not even gonna begin on how unplayable the lag compensation and balance issues made MW3...
TL;DR CoD4 and WaW were the best and MW2 and 3 make me want to burn things, things that aren't meant to be burnt... because they're absolute trash and a joke to the series
But other then CoD i'm thinking that the DayZ standalone could be a massive let down, mainly because it's taken so long and i don't think anything can quite capture how interesting, fun and new the elements that DayZ originally brought to the table were, but i'm definitely gonna still pick it up.