Blizzard admitted they screwed up the tuning between 10/25 way back in 2008. That's why you'll notice Ulduar 25-man bosses have 5 times as much HP as their 10-man counterparts instead of 3 times in tier 7. Their abilities also tend to affect a greater number of players on 25.
Ok I will walk it back, that is actually a reason to have lower health.
Although in my raids, given I'm on the more casual side, I would prefer no one stand in slime because:
--It messes with Prayer of Mending bouncing between the tanks
--It messes up parse analysis for healing
--I want the DPS to know that if they get up to #3 on threat...they gonna DIE.
Raid bosses will always be very similar so long as encounter design requires DPS to always be pumping 100%.
Analyzing parses doesn't really come off as casual side imho. Also don't blame melee for tanks' issues. If your offtank can't keep up with threat and your healers can't top off the tanks then sry there are bigger issues.
We three tank the boss and have four healers the entire raid. Our MT is actually offtanking because he has the highest health pool. This minimizes the issue with DPS pulling aggro. Maybe this is helpful.