Trace width and spacing
On the Internet there may be 'specification wars' between PCB manufacturers. Track, space, drill holes and vias are a differentiators. Some companies might be conservative and quote figures which are well within capability, and others may be at the edge of their capability. 

Laen of OSHpark has run some tests on several services offered over the Internet, designed to reveal manufacturing defects. The test PCB's used the finest tolerances offered, and the boards failed. IIRC some percentage of the boards were supposed to have been electrically tested, and were passed, but you should double check that with Laen.

Advice I was given by some experienced designers is: avoid using the smallest track, space, annular ring, holes and vias offered by your PCB manufacturer, at least until you get comfortable with their capability. The general advice was use an extra couple of mil (thou) above the PCB manufacturer's limits on track, space and annular ring, and one or two drill sizes larger on vias and drill holes to increase the likelihood of it being made correctly every time. Leave an extra margin around board dimensions and routed holes because defects which short copper surfaces are awful to debug.

Other advice included: 


leave as much copper as you can on the PCB; you paid for it. More
importantly, the waste chemical etchant is a material which requires
careful handling as a pollutant, so try to minimise the amount of
copper removed, and so minimise waste.
make annular rings around holes slightly wider, and tracks slightly
wider if the board is to be soldered by inexperienced people (e.g.
beginners). Beginners make more mistakes than production trained staff.
For example they often put parts in the wrong holes. When they remove
the  part, they are likely to overheat a small pad, and pull it off too,
ruining the PCB.


Edit: I defer to Andy aka, and other experienced community members on PCB design.

The advice I have is try to ensure 'islands' are connected to a relatively continuous ground plane using vias, and not disconnected. This is especially true around low analogue voltages and higher frequencies, where 20MHz is definitely 'high frequency'; I do MCU boards, where most high frequencies are internal to the MCU or communication interfaces e.g. USB where I take special care. Disconnected areas which are not in those contexts I might leave, especially if I am worried about heat dissipation, though I rarely have disconnected areas.

However, I do get experienced people to give my PCBs a review. You might consider asking for a review here. I know I feel more confident after someone I respect has a look at my PCBs, not least because they might question an assumption which is no longer valid.