I'm not going to talk about a particular bad instance. I don't think that the same instance is most hated, or worst, or whatever for everyone. I think, instead, that there are things about instances that contribute to their "suck factor." I'm going to try to list some of the factors and instances that are strong candidates for sucking in that way.
- Confusing Layout - It's hard to find your way in instances that are essentially circular. I can generally keep a pretty good mental map of an instance until I run in circles.. If those circles just happen to be in the form of ramps or staircases, you're quickly descending (hah! I pun) into the realm of circular maps with multiple overlapping levels. The sort of place that makes even the most die-hard orienteer crazy. You need look no further than say Gnomeregan or Sunken Temple to find this sort of layout. For an example of non-circular but nonetheless confusing, look to Mauradon. Wailing Caverns is also sort of weird in a semi-circular layout.
- Too Long - We admit it. We're spoiled by instances that can be completed in two hours. We don't like spending our lives in a five-man. It's definitely high on the suck-fest quotient to make us go into long instances more than once to complete quests. This is most notable for the non-winged winged dungeons. I'm speaking of those places where you can go to various areas of the instance once inside but that are not separate entrances themselves. Look at instances like Blackrock Depths which has several ways to go all of which can be a longish dungeon crawl all by themselves or the two distinct halves of Mauradon. Even some low level instances (*cough* Gnomeregan) can be might long at level.
- Midde of Nowhere - These dungeons lovingly crafted are far from flight paths. While these can vary highly by faction, some offenders for the Alliance are: Mauradon, Dire Maul, Sunken Temple, and any of the three in The Barrens (Razorfen Downs, Razorfen Kraul, and Wailing Caverns). In fact the Razorfen instances are not close to Horde Flight Paths either. If it weren't for the transporter in Ratchet, Gnomeregan is way far for Horde and The Deadmines is a trek as well. Uldaman is inconveniently located for both Horde & Alliance, but I think Horde has it worse.
- The Graveyard is WHERE?!?!? - Wipes happen. Which means one (or more) of you is going to have to run back and accomplish the rezzing. Blackrock Depths and Blackrock Spire are both a HAUL back to the graveyard at Thorium Point. Before Blizzard removed the auto 6 minute release, it was nearly impossible to not run if you died in BRD. By the time someone got back, you had almost always autoreleased. And while not a five man, the starter raid Zul'Gurub fits this bill. It seems that with Outland, Blizzard mostly learned this lesson, however, the ghost walk to Coilfang is still tedious.
- Why can't I go there? - Only two instances, and admittedly low level ones, are located inside capital cities. But Alliance can't easily go to Ragefire Chasm and likewise for Horde and The Stockades. By rights, these should be just outside the cities making them visitable, albeit challenging on PVP servers.
- Poor Crowd Control Options - Anytime there is a dearth of humanoids, CC options become fewer. Usually this is made up for by substituting beasts or undead which still allow shackle & sheep (or hibernate). However, sometimes even those are passed over in favor of other non-CCable suckfests. Look at Mauradon (with its plants and elementals) and Dire Maul (with its plants and demons). Luckily, two often underutilized CC classes (Hunters and Warlocks) shine here. But your mages will quickly top threat meters without sheeping to attend to.
- Poor Cost to Effort Ratio - With the advent of the Burning Crusade, many dungeons fell quickly into this category. The effort of doing a level 60 instance for substandard blues just became not worth it. Doing just a few (starter) quests in Outland will give you rewards that FAR outshine anything in there (even the epics). And here the list is long, Blackrock Spire, Scholomance, Stratholme, Dire Maul, and even BRD and Sunken Temple. I've even heard it said in my guild that raids like Zul'Aman fall into this category.