Someone in this thread mentioned Supernova as being a difficult mission, and it definitely is if you intend to use Banshees. Funnily enough, this mission is easier than easy on Brutal by A-moving Thors. Seriously, give Thors a go on Supernova. You'll slap yourself for the difficulty you had on this mission when you first played it.
Or like Haven's Fall. It's a waste of units to try to save the settlements. Post up about 5 Siege Tanks on each entrance to your base, some Bunkers and Missile Turrets. Nothing gets through, and you can build your Vikings to critical mass instead of constantly having to replace them after saving settlements. Once you have critical mass of Vikings, you systematically push out and destroy the settlements one by one. As an amusing bonus, your Siege Tanks will have 1200+ kills by the end of the mission.
I agree though, for regular RTS players, Brutal is way too easy. Hopefully in HotS there is a little less of being spoon fed a new unit each level that happens to make that level 20x easier. Brood War had true difficulty in that, even though they gave you new units to help you, you still had to grind through huge enemy bases that put up a fight no matter what you used.