It's indirect, but when I run sandbox I generally want:
- Easy prep monsters
- Know the system well enough to reskin on the fly
- A power curve that lets the characters get in over their heads and then out again
- Elements that won't throw balance out of whack if found out of order (magic items and rituals in 4E)
- An adventuring party that covers all the bases (see 4E skills)
And so forth. I think 4E stacks up pretty well on those points.
Thus sums up my experience with recent conversions of a 3.x/Pathfinder sandbox mini-campaign.
There are a couple issues that I see with 4e and sandbox though. It has a MUCH narrower tolerance for off-level encounters. I recall in a 2e game a few years back throwing a Hill Giant at a level 3 party. It was fun and challenging for them. One character IIRC ended up squished flat, but the players loved it. I don't think you could run that encounter in 4e out-of-the-box. I'm sure you COULD do it, but you'd have to specifically design the whole encounter to work with low level PCs. In 2e it was just a regular encounter in my sandbox that the party happened to blunder into even after they saw the warnings about not going there.
In my experience, the the narrow tolerance for off-level encounters is based upon a fallacious paradigm perpetrated from the beginning. I am not saying AbdulAlhazred's statement is fallacious, I am speaking more to the mentality in general.
It is as "appropriate" to have the PCs blunder into an off-level encounter in 4e as it is in 1e or 2e. The problem is that since the beginning of 4e, and to some extent the bulk of the 3e life-cycle, DMs were told that they needed to make encounters level appropriate because that was "more fun". It was almost as if they were so focused on mechanical balance that they decided that players couldn't make intelligent decisions like "run away!" or "holy-crud that was tough, we're gonna get pounded if we stick around. Let's regroup." This was almost a staple of 1e.
For the most part, it was all mindset, not mechanics or system. In 3.x/4e, the game world existed FOR the PCs. In 1e, the world existed DESPITE the PCs. And, as far as I am concerned that is all about DMing style and the implied gaming contract that is agreed upon by the group.
This is becoming more and more apparent as I more fully sandbox my 4e game. Initially, a dude whose experience was almost all 3e/4e just charged into every encounter assuming that he likely won't die, and if he did it was luck of the dice, not because it was just plain stupid.
Fortunately in 4e, there is some room for error. In 1e, if you guessed wrong, one shot can kill you, but in 4e you can get clobbered in one hit and still survive to run away.