What things to consider when designing a single layer board?
I've designed around 8 or so single sided mixed SMD/through hole PCBs (most with microcontrollers, one with some radio modules, and a couple with a low speed gate drive).

If you're doing anything that requires a really good ground plane you should probably stick with the two layer board at a minimum. Expect to put a lot of effort into getting your ground planes to be continuous with a single plane of copper. I've heard it said that single sided designs without jumpers are a kind of holy grail for PCB design. The ratsnest on your pcb design tool is actually a very good indicator whether or not your design will be easy to route on one side. Part layout and grounding will be your most difficult challenge with these designs. You'll want to actually trace out the likely path your ground currents will take (or use something like Hyperlynx PI if you have access to that kind of tool).

That said, I've successfully mixed through hole and smd components to give myself a two sided board with one layer of copper (through hole on top, smd on solder side). These board were all milled rather than etched and didn't have any solder mask. In fact, I tried my hardest to only go with single sided boards because the people who operated the milling service I was using were not so great at aligning the two sides (and the through holes weren't plated). I guess that's the price to pay for $0.07 per in^2. If you're going with a standard board house, their two sided rules (trace width, hole size, etc) should apply all the same. From their perspective, a single sided board should be the same as a double sided board unless they're doing something wonky.