Just a couple of comments from the peanut gallery : I'm not as experienced with DDO threat and agro and the intricacies of combat as many, but I wanted to sound a word of caution:
One of the reasons I left a previous (very popular) MMO is that I found there became almost an entire religion built around the mechanics of agro and threat (and around the AH too but thats another story.) I definitely agree that knowing how things work is important, but there were two things that bothered me about what happened there.
First, it seemed like if you were not a caster with CC you were expected to tank, period. So the game seemed to boil down to Crowd Control, Tanking and healing. And, each build could only perform one role so until you built several alts you were trapped in a role, period. One of the things I've liked about DDO is there seems to have been a bit more loose philosophy allowing many different builds to try many different roles. Heck, I became so tired of Tank or CC or Heal or don't play. I think the concept of tanking, although apparently necessary, seems to take away a lot of the fun of these MMOs. I don't remember "tanking" from PnP like it seems to be embodied online.
The second outcome was that huge and vast tools were developed around the concept of agro and threat, and a large portion of the game just became a numbers contest of who could dish the msot threat. If you couldn't dish out -the absolute most- DPS or threat, you would have a great deal of trouble finding a group for any higher level runs. While some of this is normal and expected, it can be taken too far, and I'd prefer DDO be careful when addressing threat issues.
Just me, peanut gallery, for whatever these comments are worth, or not...