You (and your palyers) might be stuck in a "3e-ism".
"How Healthy You Are" is not determined by current hp totals. It's determined by current Healing Surge totals. From the DM's perspective, that means "challenging the party" doesn't mean running them down to 0 hps, it means running them down to 0 healing surges (and 0 daily powers).
So it's entirely possible to challenge your player's PCs - even as they keep their hps at 100%. Challenge the party by doing what the word challenge means: give them tasks to accomplish that require skill, effort, and luck -- and don't worry about their hps. Don't obsess about hps.
Hmmmm.....
Is it possible that "an exciting fight" means something other than "reduce the PCs to 0 hps"?
This is interesting.
Yes, I feel that for a fight to be really exciting, there needs to be risks involved.
Okay, so this risk might not be running out of hit points.
You're saying the risk can instead be running out of healing surges.
But 4E isn't designed for any single given encounter to run you out of healing surges.
Just as a fight where the monsters can only expect to scratch your surface by hoping to do perhaps a dozen points of damage is dull, a fight where the monsters can only expect to make you use up one or two surges is dull.
My problem is that I have neither the time nor the patience to prefix every interesting combat with a string of easy fights whose sole purpose is to lower an abstract number like "8" to perhaps "3" before the real deal starts.
At least in 3E, going low on resources meant your options changed and were reduced. Having fewer surges don't mean a thing in 4E, only running out of them does.
And thus I'm thinking I might start every fight at effectively 3 healing surges (one second wind, one healing potion and one Healing/Inspired Word).
Not only will this mean I'm getting to the beef early, I'm getting there every time. And not only this - but I've solved the 15 minute adventuring day too (because your three heals replenish after each fight).
The only remaining issue is what to replace as an attrition mechanic. Of course, there needs to be a rule that explains how you can't go on forever.
It is in this light I came to think of disabling powers for being downed. (Not only would this be more reminiscent of 3E and its spell slots; but it sure beats trying to get worked up over having "only" a 3 on your sheet where you previously had an 8)
(Now I'm collecting my thoughts from all over, from several different threads. I'll probably summarize and post something up later)