- Repeating the same dungeon over and over
- Too many trash pulls and not enough bosses
- Encounter variety and mechanics are always the same
As for trash, I like trash mobs. They are not there just to provide you with loot and fights. In fact, they aren't even primarily there for that. They are there to provide a real world: to set the atmosphere of the dungeon, and to provide a setting that makes the dungeon more credible. Take, for example, the instance Keen mentions, Utgarde Keep. And forget how the dungeon finder teleports you in, so that the instance seems to be suspended in space and time, rather than set in an actual keep in the Howling Fjord whose entrance you reached by battling through the Vrykul to the north of Valgarde. You fight your way into the keep. It is full of Vrykul. Of course it is. It's their keep. Should it just have a handful of Vrykul and three bosses? Keen's view seems to be that the sooner he can get through the trash and onto the loot piñata bosses, the better (and sadly, Blizzard seem to be siding with this kind of argument: on the Timeless Isle, you don't even have to kill bosses to get your loot. It showers on you from the heavens). I can't say I'd find such an instance interesting, even the first time through. The trash make the instance come alive, they make it credible. Utgarde Keep is a real keep populated by vrykul soldiers, blacksmiths, guards, and so on. Without them, it would be empty.
But credibility and atmosphere aren't the only benefits of trash mobs. They also provide a cadence and rhythm to instances. A warm up to the boss. A way of building up the tension for the boss fight, and a counterpoint to it. The trash mobs, of course, are easier than the boss fights, but the best trash also provide a challenge of a different nature: the trash in the Lost City of Tol'vir or in the Halls of Origination required teams to plan pulls, to communicate, to practice small pulls, to practice crowd control and to practice focussed fire - something that came as a shock to many adventurers after their previous practice of silently rounding up everything in sight and AoEing it down. It came as such a shock, in fact, that they whined to Blizzard for a nerf. But while those instances still required teamwork, they were great, precisely because of the trash.
Of course, Keen is quite right that 5 pulls that are all the same is dull. Trash needs variation, and the best trash does have it. Meeting the same trash over and over again isn't great design. I'm thinking here of the two bridges between Jin'rohk and Horridon. Perhaps one would have been sufficient! But the best trash adds variation and forces the instance team to respond in ways that boss fights don't. When encounter variety and mechanics are always the same, the fun goes out the window. Part of that is the fault of overgearing. Once we're overgeared for an instance, we can often reduce every trash pack to the aforementioned tactic of round up and AoE down. Part of this is also our fault. We whine when unfamiliar mechanics are introduced that take us out of our comfort zone, and Blizzard is ever ready to nerf whatever we QQ about.
Finally, Keen suggested:
One of the biggest changes I would love to see in Themepark dungeon design would be to add way more bosses. I think a dungeon full of nothing but boss fights would be fantastic
I hate to say it, but it was tried and found wanting. The Trial of the Crusader was exactly this. Blizzard listened to players like Keen who said they hated trash and only wanted boss fights. So they designed a raid that was only boss fights. And nobody liked it. It felt thin and hollow, and actually boss-fight after boss-fight without end, proved to be as dull as trash after trash with no boss at the end of it. Adding way more bosses simply proved that the highs of a boss-fight need the pause of trash to make them feel epic. And the most innovative fight of the lot there, the faction champions, were the least liked (again, because they moved adventurers out of their tank-and-spank comfort zone). Another instance that was almost (but not quite) trash free was Blackwing Descent. The boss fights in this instance were fun, but there was no sense of being in a real credible world. Without much trash to provide a context, there seemed to be no reason for any of the bosses to be there. Finally, the most memorable thing about that whole instance is the elevator that killed more adventurers than the trash did.
So where is trash used well?
Kharazan is probably the most memorable. The trash mobs bring the place alive. The same is true of the Lost City of Tol'Vir, where the trash also added great variety to the encounters.
And the worst?
The Throne of Thunder has just far too much trash. But the worst has to be the trash-free Trial of the Crusader.