Interesting stuff here.
I would think (not having played/read
4E) that for dungeons, if you want an older-school feel, you should not count random encounters toward "milestones". Random encounters are supposed to be things you avoid because they just wear you down.
I like the idea of critical hits reducing max hp. That's an easy way to put dire wounds back in. You could say that max hp increases back to normal at the rate of 1 per day (or 1 per day per level, or whatever); however, you could also say that any magical healing is able to heal you up to the true maximum (so warlord powers and resting don't help much here, but magic stuff and cleric powers do).
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