Some things I'd love to be able to see easily
1) Did my mitigation attempts make a difference? (Or did I do them 2 seconds 2 early and miss the burst damage)? All mitigation and avoidance changes need to be measure over a couple of fights to get a legitimate average of how it change.
2) When someone else grabbed agro on a mob was it a) me being stupid on my attack rotation b) s/he taunted (oops sorry!) c) some other thing. Even just seeing threat over time and being able to dive into the log at where my relative threat dipped would be helpful. I don't know of any current logs that will show tps or threat gained. I would be interested in this as well.
3) Things like did my blood draining enchant actually make a difference or just cause overhealing. Blood Draining itself will never cause overhealing.
4) Some other useful information that I probably haven't thought of as important but should have. I compare myself against the other tanks in my guild when we have a fight where we are taking the same blows. Like Kolo and OT'ing 25 man patch. I compare hits avoided, largest hit took, and dmg mitigated... you can do this from WWS and WoL.