A recent post by a Champions Online forum user drew Cryptic team member Balseraph out of hiding and he posted some official insight on defenses in the game.
http://forums.champions-online.com/showthread.php?t=16333"Small corrections to Regeneration - it's actually a pretty complicated beast.: it does start off with fairly good healing at the first hit, and ramps up as you get damaged, capping out at 3x its initial heal rate. One of the tricks to it is that the rate of regeneration isn't based on your hit point level, but rather on the amount of damage you've taken. In other words, even if part way into a long fight an ally heals you up to full, it keeps going at full power.
It automatically "forgets" damage as time passes, though... about 1 attack worth every 4 seconds or so. It takes 30 seconds of time out of combat to fully "reset".
The healing from regeneration is *not* a percentage of your life; it's a flat amount that scales up with your level. The active button burst O heal, however, does scale with your HP (technically, your CON), but has the downside of making your regeneration "forget" a lot of damage.
The intent is that the active button play for Force Field and Regeneration are opposite patterns. With Force Field, you want to press the active button every time the cooldown is up, as soon as it's up, which means making sure you have the energy to push is (and thus holding back a bit on dps).
Conversely, with Regeneration, the active heal button is really cheap, but you want to push it as RARELY as possible, since it slows down your passive regeneration substantially.
(and if you buy them both, even though you can only have one passive at a time, you can use both actives, leading to some interesting combinations...)
Yes, graphing that might look odd. =)
For what it's worth... no, these defenses don't work remotely like anything in CoH, CoV, WoW, or really any other *MMO* that I'm familiar with; they're all designed to be unique and encourage different play patterns for "tanking".
There aren't any "Reactive" heals in the game. Regen heals on a tick, but nothing heals "per hit" anymore (or if there are, they're vestiges that will be removed by launch). Even at "full power", regeneration is returning ~slightly~ less than 75% of expected incoming damage vs. a single foe equal level, equal quality foe. The rate of regen doesn't scale based on the number of attackers, just the rate of damage incoming. The active button is your "scaling" mechanism - if damage is coming in twice as fast, then the "penalty" for pushing it (i.e. damage "forgotten"

is only half as bad (since regen heals on a fixed tick).
Supervillains (and worse) are of a higher "quality" level (i.e. do more damage per hit) than players, and as such regen will be healing a correspondingly smaller percentage, thus strongly necessitating healers.
As to "Stacking"... well, that's just not an issue. There is one Regen power. It's called "Regeneration". It doesn't stack with anything; you can only have one "passive" defense type functioning at a time. You can have as many active (push button, get a benefit, button goes on cooldown) as you want / can slot.
I've put a stupid amount of work into tuning all of the defenses in a similar vein, so that they balance well across a wide variety of fights. There still more work to do, of course, but I'm feeling pretty good about where they're at."