Can you think of any others? Is the suggestion more accurately 'Identical zones do not stack' such that you don't have two Consecrated Grounds or whatever out at thet same time, or is there something wrong with different zones with similar purposes stacking in some fashion?
Like if someone had a daily zone that healed people for a few damage at the start of their turn, and someone else had a different daily zone that healed for a few damage at the start of their turn, are you arguing those shouldn't stack? Or that someone in a hunger of hadar and wall of flame shouldn't take both?
Hunger of Hadar and Wall of Fire (I assume you meant that) stack according to the rules because Hunger of Hadar is a zone, and Wall of Fire is an area.
If zones overlap and impose penalties to the same roll or score, creatures in the overlapping area are subject to the worst penalty; the penalties are not cumulative. Similarly, a target in the overlapping area takes damage from whichever zone deals the most damage, regardless of damage type.
Although this uses the word area, it is in the Zone section of the rules discussing zone "overlapping area", not Area effects.
The rules should, however, be consistent. Hunger of Hadar and Wall of Fire as two overlapping "areas" should not stack because two damaging overlapping zones do not stack. IMO.
The rules should also deal with time. Damage when entering the zone or starting in the zone like Hunger of Hadar is different than damage at other times, like Stinging Cloud at the end of the turn. There are, TMK, no explicit rules on this. One could use the rule above to only have the greater damage apply (which becomes a bookkeeping chore, was Johnny attacked earlier in the turn?), or one could say "the other damage is not occurring at this time, so only the one occurring is the one occurring now" and have both zones affect the creature.
Getting back to your other question, I think that the any healing zone effect should not stack. What is so special about healing compared to damage? If two powers heal (at the same time as per above), then only the greater healing should apply, just like the damage and penalty rules. The issue that comes up is powers that heal at the start/end of the caster's turn. I am not aware of any such powers, but I suspect that they might eventually exist. Most zones are "when a creature starts his turn, or ends his turn, or is hit", or some precise moment in time, not one dependent on the turn of the PC (which with two PCs, would have two different points in time).
The thing about healing in 4E is that it is now way out of control. The Cleric in our group increased her healing by about 50% once Divine Power came out (and she did not even take Astral Seal or Healer's Mercy). Items, Feats, and interacting Healing Powers are all over the place now.
Having said that, I think that there are too many different synergies (e.g. All Soul's Ball and Soulshock Field both allow a healing surge when an enemy goes to zero hit points, but one allows one surge and the other allows one surge per ally; "Hey, my zone didn't do anything") that can happen, so I do think that the easiest to adjudicate rule is to prevent two identical zones from stacking. Any rule more detailed than that might start getting more difficult to adjudicate consistently.
It would be nice to put the brakes on zone healing though.