I got thinking... It seems the game is worse off when the characters have access to "too much" healing.
Disclaimer: I did not read all the posts in this thread, merely the original post and skimmed some of the subsequent ones.
I agree with the basic premise as a generalization - there is such a thing as too much healing to the point where it becomes ridiculous. The DM just has to throw tougher/trickier/etc things at the PCs and as the PCs go up in level, it just escalates.
It seems your PCs have taken an extreme, rather than one or two being specialized, everyone is dabbling in it. *I* would find that boring as a player, but for whatever reason they all chose to do that, which is fine. Just have to figure a way to make it work.
I'd stop giving out healing potions (if you were already) - frankly, they don't need them. And then I'd worry about enemy tactics that were focused on a) conditions b) high damage output c) recurring damage.
Get them caught in areas without line of sight or line of effect (darkness, lots of winding walls in narrow corridors) - some powers can't work like that.
And change monster tactics, inflict moderate damage on lots of PCs at once (burst effects and such), not just one time but multiple times. And same for recurring damage - they can heal up but potentially will still have more damage to take the next round.
I recently killed a PC that (to keep this story simple) was nearly untouchable leading to a bit of overconfidence. But the last session was stuck with a lot of damage and without line of effect for healing from allies and he went down fast.
Beyond stuff like that, I would find it boring (but again, that's just me if I were playing in such a group).
But having said that ...
(Sorry, we don't have time to string out three or four initial boring-but-resource-depleting small fights before arriving at the real meat of the scenario).
I think therein lies the crux of the issue.
The system was designed for multiple encounters, generally leading up to a climax. If you're taking out the multiple encounters, it turns in to a one fight a day thing, with no difference between encounter, milestone, and daily resources (thus action points, magic item usage, daily powers, encounter powers, healing surges, magic item daily powers, and a couple other things all get lumped in to one 'group')
If your group doesn't have time for multiple encounters, perhaps either have the smaller encounters one session and then the climactic fight the next session - just be sure to keep good notes in between as to who has what left (surges, powers, action points, etc).
Alternatively, reduce the PCs number of surges per day by 1/2 or 2/3 or 3/4 to mimic in the system the equivalent of having gone through several combats already.
So, yeah, I'm basically agreeing that you can get too much healing. But based on what you said, it seems like your group has a different way of playing (which is perfectly fine!!!!) than the way the system was set up.