On the issue of healing on a natural 20: a comment from another poster on another thread (in CircvsMaximvs) led me to realize that the hit point recovery on a natural 20 is only in the "Try It Now!" section. The actual 4e rule might be that on a natural 20, the character actually stabilizes (no further rolls needed) and may use a second wind, if he has any left.
Of course, if you don't like the idea of second wind in the first place, you're not going to like this any better, but otherwise, it's no longer an opponent "healing" a character by wounding him - it's the character being able to draw on his inner reserves and recover enough to keep on fighting despite having being knocked unconscious earlier.