Critique my SMPS buck design
Good first stab, just some thoughts:

1) The R1 / R2 feedback voltage divider has some really long traces and paths to ground.  Anything you can do to keep the FB traces short will improve regulation stability and the ground used should be tightly coupled to the IC ground.  I'd put R1 and R2 directly next to each other and put em where the small signal diode is, moving the diode over to where R1 is now.  Ground R2 to the IC ground via (its low current).  The LED can go pretty much anywhere and trace length doesn't matter so just move it to fit.

2) The input caps, C1/C2 and the catch diode D2 should be very tightly coupled, their ground references need to be very very close to each other.  I'd flip D2 90 degrees and move it over next to C2, ground them all right there, you should use more than 1 via for this ground.  What i normally do for this is a little copper pour on the surface that connects to all 3 pads (D2/C1/C2) and extends out enough to plop 3 decently sized vias in it.

3) the output filter cap C4 is in a really odd place It should also be quite tightly coupled to the ground of D2/C1/C2, move L1 over and you can probably put it right next to D2 and included it in the little ground pour I was talking about in 2).

4) With the above re-organization you can move C3 over just above/to the right a touch of the IC which will get rid of the problem you have with the input supply trace having to fit between C3's pads, as drawn right now that will short out, the trace is way too close to the pads.

Keep in mind that your critical path in this circuit, that is the loop where there will be high current AC and DC spike is from the input -> D1 -> C1+ -> C2+ -> ICin -> ICsw -> D2+ -> L1 -> C4+ -> C4- -> through other grounds back to C1- 

Your goal is to minimize the loop area and the resistance in that loop, which moving C1,C2,D2,C4 as close as possible to each other and the IC will help with as well as tightly coupling their grounds through a small pour on the surface with multiple vias to the ground plane.