Does not sound like you gave the current game much of a try at all. Did you even hit max levels in the expansions you skipped out on? Because some of the content in MoP and WoD was damn good. The raiding in MoP and WoD was good, the leveling aspect of WoD was pretty good.
I know I'd get bored to tears on Vanilla again. The leveling content would pose no challenge other than just a time sink and the raiding is very light on mechanics compared to what people are used to today. It is great that you are enjoying it but your experience likely wouldn't be typical if Blizz put out Legacy realms.
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Sure Zalek, but couldn't it also be said that the same friend could get carried in the same exact fashion in Vanilla? I know it is true because guilds had to bring in new people all the time to gear them because of roster turnover being even worse because it was 40 man raiding.
But still your friends experience with making an alt on a random server and being fully raid cleared in a week is NOT typical. Most people wouldn't even get in the door in heroic without prior achievements to link or some sort of proof that they could do the content. Other likely scenarios is people would inspect him and decline him. Heroic content isn't THAT faceroll because go and look at the numbers of guilds that have completed the content. It isn't like every single guild just walks into heroic and the bosses fall over and hand them free epics. You aren't being realistic and you haven't experienced it for yourself.
If your friend could do it why don't you go and try it and come back in a week and update me on your progress.