Since Spiel has asked for some input, here are some things that I've grown dependent on in the way of Unit Frame features:
- Range. I like being able to see who is within range of me in my raid/party.
- Show only those things that I can affect. I don't need to see a million and one icons. I just need to know who is affected by things that I can cure, dispell, cleanse, or otherwise remove. Hue/Color changes to the unit frame background would probably be the best way to represent the various afflictions (Green for Poison, Yellow for Disease, Orange for Curse, Blue for Magical, Red for Aggro).
- Show healing in real time. Pitbull managed to be able to show me the amount I was going to heal somebody for in relation to their real hit points. (Meaning, if they took damage, the projected heal amount would move accordingly) ... not so much a necessary thing. Just nice to know so I don't over heal.
- Click casting. I find the old way of targeting heals, buffs, and debuffs to be unsatisfactory as I do not wield the necessary skills to swap targets fast enough or with the required accuracy.
- I like my raid and party frames to be organized in such a manner that at a mere glance I can tell who is what class, which group they belong to, how much health/power they have left, and if they have aggro.
Your HUD presents all the necessary information in regards to enemies so I'm not to sure what else would be required.
I'm sure that's asking a bit. But that is what I've grown dependent upon. I hope that it gives you some ideas or at least adds to the weights of certain planned features.