Actually, there were (still are) some of these services floating around. Sentry Totem was doing class coaching till they closed their doors and I think Skill-Capped does coaching for PvP (paid teaching though).
I can understand but for us the current comparison (at least on AMR's inforgraphics) is that our best players hover around the 75th to 50th percentile median whereas the bad players are like in the low 20th percentile (or even lower). I suppose at some point, we're just going to have either cut the team down or be satisfied that we won't be able to progress as deeply as we'd like.
Others can correct me if I'm wrong, but log reading seems to be tied very closely to understanding how a spec works. For instance, I know that frost mages seem to highly value the 1:1 ratio between their procs and the instant spells cast for those procs (i.e. FoF -> IL, BF -> FFB). Meanwhile, back in MoP, for restro druids it was having high Harmony uptime (not sure if this is still current for WoD). And for brewmaster tanks, 100% shuffle uptime with talents like Serenity.
In the end, I usually have to find sources regarding how a spec works to then be able to analyze logs for performance. I still can't figure out how read warrior logs (Fury specifically).
Hmm... so then are there ways to inspire bad players to become motivated to improve?