Way back in late Burning Crusade, my Warlock moved from a decent guild that raided to one of the top raiding guilds on the server.
I had always been in the top 2-3 for damage in the previous guild.
But, even taking into consideration the relative lack of gearing, I wasn't doing that great for DPS with the new guys.

A couple of them looked at logs, I believe it was WWS or something similar then, not worldoflogs.

It turned out, I was using Lifetap preemptively when low on mana, to prevent myself from running out of mana... instead of burning entirely through the mana reserve, and then only tapping the minimum amount to cast the next spell.

It could be something small, that can make a fairly big deal in the long run.