For designer[some apply to level editor]:
1) I second requests for cut-n-paste(aka c'n'p) and 'undo' operations within level.
2) Transformation of selected object:
2.a) rotation (we have one in level editor!),
2.b) flip (vertical, horizontal)
2.c) zoom(can be useful for W-------W longcars
['W' - wheel, '-' - rod])
2.d) change wheel types and rod types
3) Save selected object in some hmm PartLibrary, so in other levels player can use saved object again
4) Place objects illegally(of course with such objects 'start' button must be disabled).
Without this, usage of c'n'p and transformations will be very limited.
Also this can be handy for tuning solution: for example player needs to connect rods to top of some wheel A, but it too hard because there's another wheel B right on top of A. With suggested feature player can move A out of building box, connect rods, move A back right below wheel B. Sometimes it's hard to find a free space inside of building box.
5) Better select tool. Ideally i'd like to select multiply wheels/rods just like selecting several files in windows explorer(ctrl+click, ctrl+clikc and pair of files is selected). But button that will switch 'select single item'/'select all connected items' mode can be handy too.
5.b) When dragging rod, highlight element, to which this rod can be connected right now.
Example: player created a wheel, then started rod in another part of screen, then dragged end of rod to center of wheel. So when player dragged rod over the wheel --- highlight this wheel and its center point.
This feature can be useful in situations where several wheels placed tightly and player doesn't know exactly to which wheel rod will connect(see also 4))
6) Rod Tools
6.a) Split rod into two rods at given point (was x-----x, after tool usage, became x---x---x, where 'x' is a joint point of rods and '-' is a rod ). Useful not only for chains.
6.b) Split/merge points. For example, was W---x---x single object, became pair of objects W---xx---x after splitting.
Suggestions for screen of level selection:
7) Auto difficulty for each level: place (number of people who solved level / number of people who played level * 100%)
near each level.