The following is probably going to make me sound like a complete weirdo:
I prefer trash pulls to boss fights.
Yes I know boss fights give you the cool loot, and I know boss fights have exciting mechanics and scripted events, and I know that when you get right down to it every trash pull is the same, but that's kind of why I like them.
This might sound strange, but what I like most about WoW is the strategy. I know that the strategic element of WoW isn't terribly deep, particularly not when compared to serious wargames like Hearts of Iron or Europa Universalis and certainly not when compared to classic strategy games like Chess or Go, but none the less there's an element of strategy there, and those elements are primarily present in trash pulls.
Trash pulls are simpler than boss fights, but that's exactly why I find them more strategically interesting. Go is one of the most strategically complex games in the world, but it has precisely one type of piece, one type of move you can make, and a set of rules you can write on the back of an enveleope.
On a trash pull, you have access to your character's entire box of tricks. You can sheep, sap, shackle, root, hibernate, banish, seduce, kick, bash, silence, and generally do everything game mechanically possible to negotiate the pull. Tamarind wrote recently about an attempt we made to three-man the Blood Furnace (slightly above level, admittedly) with a Frost Mage, a Demonology Warlock, and a Boomkin. It was, as the frost mage kept whingeing, remarkably easy. The reason it was remarkably easy was that that's how strategy works. That, in fact, is why strategy is cool. It's about defeating the enemy with your brain.
In boss fights, on the other hand, your bag of tricks is rudely confiscated by the video game equivalent of an overzealous hall monitor and you're forced to play the encounter the way it is "supposed" to be played. You can't Death Grip Svala Sorrowgrave under her own sword, you can't silence Novos the Summoner with a well placed Avenger's Shield (in fact, checking his link on WoWwiki Novos doesn't even have a damned Aggro table, so you can't tank the fucker either sooo what am I supposed to do in that fight again?).
To put it another way, trash pulls are all about preparation. Proper marking, use of crowd control and so on can make the difference between a wipe and a walkover. A boss fight is all about implementation. There's usually only one way to approach it, and success is mostly a matter of stats, rotations and Not Standing In Fire.
Strategy aside, there's also immersion to consider. It's been said time and again but if there are only two trash pulls between bosses, I don't feel like I'm exploring a fortress or a ruined city, I feel like I'm, well, playing a video game. I don't want every single instance to be a vast, sprawling monstrosity on the level of Blackrock Depths, but I want them to look like the things they're actually supposed to be. My favourite Northrend instance is actually Violet Hold for that very reason. Sure it's the model for the new "all bosses no waiting" school of instance, but it feels like what it is: a magical prison complex under attack by the Blue Dragonflight. Trash pulls make you feel like whatever you're attacking has a function beyond just spitting loot at you, and difficult trash pulls, where you can't just run in and lol AoE noob make you feel like these places are actually defended.
Perhaps we'll get really lucky, and Blizzard is going to pull a bait-and-switch with the Colosseum, so that after the jousting we'll collapse through the bottom into the ruins of Azjol-Nerub, and find them swarming with Scourge and Faceless Ones.
I kind of doubt it though.