I did several Mythics last week.
In general, they aren't that difficult with a tank that pulls safely. You know, pull back to the group so other groups don't get accidentally pulled, use crowd control occasionally, etc.
Trash can be difficult, especially if you get multiple packs at once. I would rotate cooldowns for the trash packs to make life easier. Avenging Wrath - Holy Avenger - Blessing of Sacrifice - Holy Avenger, one per trash pack, was a good pattern that made healing easier.
I strongly recommend Beacon of Virtue for Holy Paladins as well. There's a lot of group damage going out in heroics.
The only bosses that I found unusually difficult were the Triad in Waycrest Manor and the end boss of Shrine of the Storms. Triad you can jump instead of moving, and using Heroism on the second witch got us past that.
Most of the bosses are mechanics checks. Do the mechanics correctly and the fight is easy enough.
One trend I found is that a Brewmaster Monk tank is bad sign in a group. I don't know why, if they are underpowered, or if it's a difficult class to play, but all the dungeons where I had a Brewmaster tank were a lot more difficult than they needed to be. They also seemed a lot more resistant to pulling safely than the other tanks, for some reason.