My dream trash is optional trash :P just let me skip it.
I see some people commenting on the fact that they like trash that gives a little introduction of some kind of mechanic to the next boss....Blizzard has been doing this...since I started playing the game. Trash is incredibly important before you fight a boss the first time. There is something in every trash gauntlet in Throne of Thunder that has something to do with the boss it leads up to.
As for Throne of Thunder Trash....honestly the only thing I think that is overwhelming is that the first half of the instance has a lot of trash...and the last half just gets less and less up to Twin Consorts and Lei Shen where you fight 4 mobs total that are really quick and easy to kill.
I miss traps. I remember how nice it was to have a rogue in the group in ICC who could spot where the traps were for the giant guys, so you could avoid them if you wanted to or set them off deliberately at a time of your own choosing. It was a bit of class distinction that's often missing from today's game. A rogue wasn't mandatory by any means but it was a nice bonus they could bring.
I would love to have trash that occupied a long corridor, but had a decent-to-middling chance of dropping a BoP or BoA key that let you open a door to bypass most of that trash on subsequent runs.
I like patrolling trash, creatures that don't just sit there waiting to be killed. Makes a place seem more alive, gives you latitude to discover ways of sneaking through without pulling.
I like it when there are alternating ways through the trash, where you can go one direction and face mostly casters perhaps or another way and face melees or another way and face packs.
There should be alot of trash, most of it optional. the first boss in DS was a perfect trash/boss Ratio, with a ton of extra mobs for you to kill if you wanted to farm for gear. BoT had too much trash, since very little of it was optional. I've not set foot in ToT, so I can't say but I'd love to see more optional trash.
"Reason is not automatic. Those who deny it cannot be conquered by it." - Ayn Rand
Volume wise ICC trash was about right for me. Enough time to have a breather between bosses and a relatively decent chance of actually dropping something interesting. In terms of mechanics, IMHO trash leading to a boss should give you some indication of the bosses abilities (but tuned down versions).
Like some people said, variety is good, as well as some boss-mechanics introduction. I enjoyed ToT's trash but I agree it was too much at times. Mainly between jin-> tortos and megaera-ji-kun... about half as much would've been perfect.