Encounter design: I've been really skeptical of 4e's encounter design in which each combat encounter has, by default, one creature (or trap) per party member. Here's the thing, though, encounters are defined in such a way as being separated by a short (about 5 minute) rest. So, if you are in a dungeon and hit two rooms with monsters in quick succession, those two rooms count as a single encounter. This has some interesting ramifications. Your encounter powers don't refresh between them, for one. If the players only see one monster in a room, and they know it isn't a solo monster, they get tipped off to the fact that the encounter won't be over by just defeating that one thing - they probably shouldn't spend all of their encounter abilities on it.
I'm still not 100% sold on this, but it is certainly more nuanced than I'd assumed.
Encounter pacing: One of the express design goals of 4e was to stop the trend of the fifteen minute adventuring day that consists of a single fight between 8 hour rests in which people recover spells and such. How was this done? Well, the obvious bit is that many powers are usable per encounter... and, as such, can be used every encounter, if appropriate. There are still, though, some things that refresh only with a long rest, notably daily powers and healing capacity. The gradual loss of these resources throughout a day of activity are balanced by milestones. PCs reach a milestone when they've gone through two encounters without taking an extended rest. Each milestone that PCs reach gives them an action point, which can be used to take an extra action (a pretty big deal). When PCs take an extended rest, their number of action points resets to one. Moreover, some magic items and such become more powerful once you've reached a certain number of milestones. It is a pretty cool system that adds a nice balancing feature.