Hit points/healing: Hit points represent
heroic resolve, not physical injury. When your heroic resolve is exhausted, you collapse and die. As long as you have the strength of will to keep fighting, however, you can soldier on despite injuries that would kill a lesser combatant. (This is why a warlord can yell at you and restore your hit points.)
Basically, anything with more than one hit point - i.e., a non-minion - is a
Determinator.
Encounter breaks: Pretty much everything that talks about encounters says "until the end of the encounter or for 5 minutes." Thus, I assume an encounter is roughly 5 minutes; the only reason it doesn't flat-out say "5 minutes" is to avoid giving the impression that you have to track round-by-round duration.
Tiers: I don't know about anyone else, but I'm planning my campaign such that there will be a significant break in game time, probably 3-5 years, at each tier change.
I actually like the tier system for this reason. In 3.X, character power went up at a fairly steady (and extremely rapid) rate; this made it very difficult to come up with an in-game explanation for how the characters were becoming so bad-ass so quickly. In 4E, however, there is a much shallower power curve, punctuated by a noticeable jump at the transition from 10th to 11th and another from 20th to 21st. Those transitions are natural break points.