Don't expect less trash for heroic and raid instances (src)
"Trash" mobs do serve a purpose in dungeons though and while they may not be your favorite part of the dungeon, we try and keep a good balance of it versus the rewards of the particular dungeon.
Shattered Halls may have a fair amount of trash, but I believe it can drop up to five badges of justice which is the most you can get from a heroic dungeon so it makes sense for it to be a challenge to get through.
For raid instances, it was talked about at BlizzCon and in particular the Molten Core version 2.0 that had no trash, the video should still be out there somewhere if you want to check it out.
Of the dungeons you specified, Magtheridon, Onyxia, and Gruul's Lair are specific type of raid dungeons that are designed to be quick shots to the bosses and (hopefully) the loot.
For other raid dungeons, we generally shoot for a good balance of trash mobs to bosses but we do want these big epic dungeons to be challenging too. We never follow one static idea and continually aim to improve upon things and your suggestions are welcome here: http://forums.worldofwarcraft.com/board.html?forumId=11122&sid=1
PTR: No premades (src)
There are no plans for Premade characters for patch 2.3.3.