Analysing why tank (or other critical raid members) die is a staple in raid leading, there are multiple tools that can help with that : Acheron, Grimreaper, Recount, post-raid WWS analysis etc...
Where they ultimately fail is in giving you a quick picture of what each of your healers were doing at the moment of said death : was one of your tank healers busy padding his healmeter on healing the raid (nothing wrong with that btw, until the tank dies because of it) ? Was he dead or otherwise incapacitated ? I would like to have opinions on how we could display the information in the most informative and easy-to-look-at way.
What I envision right now is something like that :
- Simple graph with tank life at the top
- Above tank life a set of icons showing all incoming events on tanks, hovering on icons show additional information in tooltip. Basically this is the exact same information than what Acheron shows, but with a better sense of the timing and scale involved.
- On the bottom : 3-4 "tracks" assignable to your healers via dropdown, those will show what the healer was casting, on which target (find a way to make it obvious at a glance if target was not the person who died).

Something like that is what I'm aiming at :
[img=http://img264.imageshack.us/img264/2396/ss9zw0.th.jpg]
On this we see :
- Tank gets healed (green triangles), damaged (red crosses), buffed (yellow star), debuffed (orange circle)
- Healer 1 : casted 2 GHeal back to back, one on our tank, another on someone else (indicated by red border), he followed with a CoH (again not on tank), slapped a renew on tank and on someone else, flashed heal the tank, again CoH (not on tank) etc... until tank death which is most likely due to not having dispelled debuff.
- Healer 2 was incapacitated very early on, he casted a holy-light, was in the middle of his next one when he got a debuff (CC ? long term silence ? just hover on the orange circle to see details), cast never landed.
Conclusion : healer 2 got incapacitated, healer 1 was a little bit too busy healing the raid, threw an emergency shield on tank when he got debuffed but could not save him. With a little bit of practice you should be able to find that out quickly from the output and do what you must as a raid leader : scream at Healer 1, find out why Healer 2 did not get back in the game.

What I'm looking for here is :
- Feedback on ergonomics
- Are there additional things I missed which would be useful as raid leader ?

I have started a tool similar to what you wish, you can check the thread here:
WowCardioRaid: Offline graphic wow log parser
Anyway, it's offline, and works with the wowcombatlog, like wws does.

Its probably possible to do this kind of tool directly in game, but I guess this will eat a lot of memory/ressources, this is why I didnt try...

Yep, I had checked out Cardio a long time ago, I admit it shows a bit on the design I'm advocating. However I think it is essential to have it work in-game, yelling at your heal team is always more productive when the shit has just hit the fan.

One thing that you didn't address in your post which I find to be quite important is the order of deaths, and whether there are follow-on effects.

One example would be doing Noth-10 with only a resto shaman to decurse, who then gets eaten by adds. You'd want to see that as the first death, and then after that you'd see the DPS and other healers dying due to the curse explosions. Ideally you could look at each death from the timeline and see incoming damage in the last 5s before death in some sort of breakdown.

I've been doing this with WWS after the fight on our Sartharion + drakes attempts, but WWS is so epeen-focused that it takes about 4 clicks to do so:
1) go into the log browser, search for 'unit.type = player AND event = unit_died'
and then for each death:
a) click on the name which takes you to the stats page for that player,
b) click on the times in their "death report"; there may be more than one if rezzes are available.
c) read the filtered player damage/heal log to see what killed them, and whether it was burst or slow.
[repeat a,b,c for each player death you're interested in]

Sometimes it takes a lot of log browsing to figure out why a player died. One of our hunters died in one of the attempts because they were getting chomped by a lot of whelps, even though they'd been AoEing for about 5-10s before that. It turns out that the AoE had started after Tenebron died, but he'd laid a second batch of eggs which hadn't hatched until the start of the AoE. The hunter happened to be unlucky enough to have her Volley be the first thing to hit the fresh whelps before any tanks had a chance to touch them.

I don't know the best way to write a tool to help with the latter sort of analysis, but at least being able to see that the hunter died from 4 different whelps hitting her would be a good start.

Filming your attempts helps.

Any time we're working on a very new boss I always film each attempt from the first night since the visuals provide something the combat log doesn't always.

Here is an idea I had...

the bars show the cast time, or in the case of a HoT, the tick.

The numbers refer to the boss event.

Well, until you find (or make yourself) such an addon directly in game, its, in fact, possible to use wowcardio "on the fly" (if you like it, of course).
I am myself using it a lot of time in this way, without having to close the game, most of the time when Acheron can't help me enough. The workaround is to simply copy/paste the current original log to get an unlocked log file, then it's directly usable for parsing, that take 10-30 sec, depending of the log size.

Neone, that image you have created is an excellent visual aide to determining what happens to specific players during an encounter.

However, I beleive that trying to process all the data and represent it in such a form would be a too large constraint on system resources on most rigs in-game.

I've taken a quick look at Post-Mortem, and will be putting it into practive during the next raid and I can see how the data is being represented without too much difficulty, but that mock-up is simply brilliant.

Directly looking at it you can see that the paladin was chaining Holy Light, and then stopped.
Why?
You would need to put in some referance point or link to another graph [or display multiple dynamic graphs depending on what healer/event your looking at] to show what happened next.

To extend on the idea [I'm unaware of Cardio's functionality as I will look at in a few hours] perhaps showing a dimmed/diff colour cast bar to denote that that healer healed someone else.

So in the mock-up, you would see a grey Flash of Light between boss event 5 and 6 to show that they healed another player, or an icon to show their death/taking direct damage [I would most likely filter AoE, but that is really encounter specific]

That's just a couple of things that I thought of when I looked at it.