A simple problem with a simple solution;
We've all done, you're working away and want to align two objects, or move one a little-ways, but you put a number in the wrong field, or make a small typo. Suddenly your prim is on the other side of the simulator, under the ground and you have no idea how to find it.

Bugger.

The fix? A checkbox in the edit window (on the tab with position/rotation etc. options) entitled "Restrict moves to 10m" (alternatively the 10 could be a text-field or drop-down list where you can choose a value yourself).
With this switched on, if you mis-type a number, the worst that can happen is that your object will only move 10m in the new direction. Or even refuse to move, because according to your new setting the value you entered is invalid; I prefer the latter (refuse to move if over 10m move) as 10m, or whatever value you chose, could still be enough to lose something.

This wouldn't stop us moving builds higher up, as you just select the build, uncheck the box and move away, then switch it back on again. I think it would make sense to have this option on by default, and maybe even have an advanced option which has it default back to checked whenever you move something (in-case you uncheck it to move something a large distance, then forget to check it again).

This way 99.9% of builder's mistakes can be neatly avoided :)