Some of these were completely unforeseen, others in retrospect were pretty obvious, but at the time not so much, and others you have to wonder how they managed to make it live in the first place. We're going to talk about them now.
Vanilla WoW: The PvP ladder
Before the ladder, there was mainly world PvP. Spots like the Crossroads in the Barrens (close to a convenient neutral port so Alliance could get there easily) and Tarren Mill/Southshore were hotly contended for almost no good reason at all besides simple factional hatred and a desire from players to kill players. All of that changed with the introduction of battleground and honor rewards, the best of which required a player to achieve a certain rank to attain. What happened next was simple - some players hit upon a means to achieve that high PvP rank, namely, play in shifts.
The ladder was abused from the moment of its introduction. People formed groups who hit the BGs together, sure, but that wasn't the abuse part. The abuse came in the form of people sharing their account information and playing a specific character in shifts, literally keeping said character in the BGs for days at a time. If you were trying to play your character fairly, you simply couldn't compete with the five people who were playing that one warlock nonstop until it had all the high ranking PvP gear, and then shifting to the next player's warrior or paladin. I knew people who tried to stay awake for two solid days doing nothing but hitting up Alterac Valley and Warsong Gulch. It was painful to watch. The ladder ended up being removed before the end of vanilla, and it was the best change they could have made.
There are a few contenders for worst decision of TBC - there's the decision to create complex attunement chains that needed flowcharts to understand them, there's changing raiding sizes to 10 and 25 and then feeding existing guilds through a 10 man raid before expecting them to do 25 man raid content, there was the decision to make the starting flying mount be slower than an epic ground mount. But for me, the absolute worst decision of The Burning Crusade was the proliferation of Badges of Justice. The Badge of Justice idea wasn't a bad one in theory - it's just that it quickly went from 'hey, here's a consolation prize so you can eventually get an upgrade' to 'we're all going to farm Karazhan again this week because it's easy badges, get your four alts ready' and by the end of the expansion, badge vendors had effectively replaced gear from actual drops for a lot of players.
In one version or another, this system has remained with us ever since. It's not one of my favorites. I think there's a definite place for some method to ensure players aren't forever at the mercy of random loot, wearing gear that's viciously outmoded because no upgrade will drop, I just don't think Badges of Justice or their various point based descendants have ultimately proved to do the job. Indeed, one of the contenders for worst decision in both Wrath and Cataclysm has been the descendant of this system.
Wrath of the Lich King: No trash!/A year in ICC
Trial of the Crusader was the worst raid we saw in Wrath of the Lich King, the worst raid design I think I've ever seen, and worse its introduction pre-empted a far superior raid in Ulduar and meant that when ICC came out, we found ourselves in a far superior raid that we then had to do for well over a year. Patch 3.2 introduced a lot of content - the Coliseum with its series of daily quests, its new five man, and the raids - and to this day, I wish they'd skipped the raids entirely. Everything else would have been fine, it would have been new content, and I think we could have stood a few more months of Ulduar. I would have liked it had the raid design of Trial of the Crusader been devoted to the Ruby Sanctum, making that more than a diversion -- in the end, though, ICC lasting a year always would have remained a problem.
I'm on record as having liked Cataclysm more than most, but I accept that it had some significant issues - starting five player dungeons were very hard to PuG and it felt jarring to players who were used to pugging everything back in Wrath with the arrival of the dungeon finder, the 1 to 60 revamp was much larger than anticipated, the 80 to 85 leveling experience felt truncated (and Uldum felt like two disconnected zones held together by duct tape) but for me, the big mistake of Cataclysm will always be Deathwing. While I understand and accept that Blizzard had a vision for the dragon, Deathwing ended up feeling less like an enemy you fought (his complete and utter disinterest in acknowledging your existence continued right up until the end -- he was still focused on Alexstrasza and the Aspects even while we were killing him) and more like a natural disaster. If Roland Emmerich designed a monster in WoW, it would have been Deathwing.
Fans of the sinister plotter of the WoW novels found no trace of that guy in Cataclysm, and I think it's a shame. A five-man where we interact with a malevolent, malicious and cunning Deathwing would have been a nice addition to Cataclysm.
Mists of Pandaria: No five-player dungeons past launch
Even The Burning Crusade had another five past launch. Wrath of the Lich King added four, and Cataclysm added five. Choosing to focus on scenarios and other means of content delivery was an experiment that, I believe, proved the value of further five-player dungeons as a content delivery system, for a variety of reasons. In an expansion that I personally believe was one of the best in terms of story content and playable experiences, the gaping hole where new-five player dungeons were not was deeply felt.
And that's the lot. If you have candidates, that's what the comments are for.