How close to us can we see with JWST, would we be able to look at our neighbor planets and/or their moons (when in the right place logically) or would things that close be out of focus? I tried looking it up, but it seems everyone is (understandably) more into how far away it can see.
Mars was a (possibly de facto) requirement. The JW attitude control system was given slew rates compatible with observations that far. Some limit had to be stated, and faster slew would cost more, either outright, or in impacts to other spacecraft elements (systems engineering), or in programmatic impacts (e. g., test requirements or lifetime or image jitter or whatever).
Slew rates are important because targets move, and JW moves. For extragalactic work etc., exposures measured in many seconds do not produce relevant motions (>PSF). For Solar-System work, motions are not just relevant but 3D- targets produce not just streaks but arcs if close enough (therefore fast enough).
In flight, part of the delay of several months (before first science light) was as-built, as-flown testing and demonstration. Ops has now reported that the attitude and slewing functions, of science-grade (not merely transit from one observing field to the next) have come in better than spec. This means JW can see closer than Mars.