Digge

You can reduce the delay between casts a bit by binding your main spell (e.g. Shadowbolt / Frostbolt) to your Mousewheel. IDK why but it feels more fluid to me than smashing buttons. Also provides you with plenty of time to scratch ur balls during raids.

I would say Druid, Shaman, Warrior and Warlock. Those classes have a high variety of abilities which you will have to decide on to use your GCD for depending on the situation and class(es) you are playing against. However, there are other things that are more important in Vanilla WoW: Knowledge about the game in general Positioning Consumables Goodies (i.E. Skull, Trinkets, Engineering items... the list is pretty long) Experience (there are people on this server that have played vanilla for 10+ years and obviously they are the best)

sap cheapshot vanish cheapshot coldblood eviscerate blind cheapshot preparation vanish cheapshot coldblood eviscerate ???

AFAIK Casts have always been on a one-roll system, at least of you don't take partial resists into the equation.

It's not implemented since it wasn't a feature in Vanilla WoW

Look here http://nostalri.us/

In the current Patch (BWL) both classes are pretty close in DPS performance, with Warlock having a small edge over Mage of played correctly. In AQ Mages are absolute beasts with Fire being viable. In Naxx both are pretty close in Performance again. Warlocks are the best DPS class for some of the hardest encounters in Vanilla, like Cthun and KT, because your manapool is literally infinite and multidotting is strong in these fights.