What makes for a really great encounter?
Arneson: That the players overcame the obstacle by wit and not muscles.
Thus spake the recently departed Mr. Arneson in an interview with Kobold Quarterly. It's a sentiment I certainly agree with. I've been writing a lot lately about the need to challenge the players directly, rather than just their characters. I enjoy more a challenge I defeat with my own brain rather than just bulling through it with dice rolls. At the very least, choices need to be made with options weighed and abandoned, or else why bother having the other players there? Choice is an integral part of what makes games fun.
Obviously, an open, sandboxy game with lots of options for "lateral" action and thought invites nearly constant opportunities for exercising your wit. A key component in my games has been including consequences. Sometimes, no matter what the players do, there are negative consequences for their actions. This just spurs them on to make more choices and look for new clever ways to aggrandize their characters while thwarting their enemies. This sort of play keeps both the players and the DM on our toes.
But that's not the only way to employ wit in overcoming a challenge. 4e, for all its faults, demands wit from players and the DM in combat. By turning fights into mini battle games, where you must constantly weigh matters of positioning and zones of support and danger, ranges, and which powers to use when, 4e has transformed D&D combat from a quick flurry of dice rolls (or not so quick, as generally was the case in higher-level 3.x combat) into a tactical challenge, a ever-shifting puzzle that is both constantly new, but also based on simple groundrules. This creates a situation that remains fresh, even as it is repeated in its basics. Simple rules that yield complex play.
The two, of course, can be combined. I prefer my combat quick and dirty, but I can imagine a system more like 4e's that also embraced a more off-the-wall sensibility.