Since every instance in Cataclysm has its own load screen, players know where they are going before they ever set foot in the instance. Some instances are harder than others. Some have awful trash. Some have boss fights that require lots of moving, and some have tons of ambient group damage. Depending on your role, which instance you want to see come up as your random probably changes. Some have that one piece of gear you need or that achievement you really would love to get; others make you feel disheartened as soon as the loading screen pops up.
I currently have two 85s; my main is a tank, and my alt is a hunter. While tanking anything but Stonecore, I am fairly happy. I hate the initial few pulls in Stonecore with Milhouse Manastorm and those damn guys that turn into rock elementals and have that disgusting AoE so much, and the trash pulls with the sentries later on also suck. My tank just runs the one heroic a day for the points so I can get my last piece of tier for feral and start picking up some of the moonkin off-spec pieces, so if I get Vortex Pinnacle every day and just blow through it, all the better.
On the other hand, with my hunter, every time I see The Deadmines I am actually happy. I still need the axe and bracers. Also, my hunter hasn't been 85 very long, so justice points are still useful to me.
What heroics do you hate to see pop up? What makes you hate this particular instance? Which are you actually happy to see?