Other things to watch out for:

Rectangularitis: fitting the features of your map neatly into the available space will result in something that looks like it was designed to fit into that frame.

Spinal mountains. Mountains tend to follow edges of continents rather than running along the centre. The edge is distinct from the coast though. You can have mountains where two land masses are colliding or rifting a mountainous spine does make sense for some islands (I live on one such IRL)

Treating technical components of a map as mere decorations. Compass roses, scale bars, graticules (grids), rhumb lines (those lines radiating out from compass roses) all say specific things about a map. Those things can easily be contradictory both with each other and your intent for the map. A map that uses these elements incorrectly will just long wrong to anyone who understands them.

Computerisms: It's very easy to give a map a "this was done using graphics software" look. Bad typeface choices, and especially using the same face at widely different sizes. Over use of glows, especially around text. Wildly varied symbol styles especially if they look like they were drawn at very different scales. Sharp boundaries between rough textures. Drop shadows and emboss effects. A strong mismatch between precision/care and neglect/roughness such as precise printing on a rough canvas texture. The key is to think "How would the imaginary cartographer have done this?".

Not understanding spherical geometry/projections. There are ways to avoid the problem by accepting some restrictions: if you work at fairly large scales (small areas drawn big rather than big areas drawn small) and avoid graticules, you can get by without understanding this. Once you need to have a larger area that's internally consistent or if you want to include those technical things like graticules, you need to learn at least a bit about this though. Unfortunately it can be rather unintuitive, and there's a lot of well meant but bad advice on the subject. A flat world avoids this problem, but has others as there are some things we take for granted that wouldn’t make sense on a flat world and would need to be accounted for (It would break most methods of navigation besides using landmarks for instance.)

Monofractals: Geography tends to involve multifractals. Approximating this with monofractals (many of the cloud, noise, and turbulence generators in graphics software) results in something that looks fake because the disorder in it is too even. All the coastlines have exactly the same amount of crinkly roughness.

Poor contrast: Light and dark, dull and saturated, dense and sparse. Good maps have contrast as it makes them easier to read. Be wary of overusing neutral middle tones. Also be wary of the inclination to even out the density of content. Real life is clumpy. Blank areas are places you can use to put things, but don't try to fill them up (strong textures, unneeded graticules and rhumb lines, etc) or you risk making it less clear what is actually important. Avoid unnecessary details so you have room for the important ones.

Good labelling is hard and bad labelling can ruin an otherwise good map. "Positioning Names of Maps" by Eduard Imhof is your friend. http://www.mapgraphics.net/downloads...es_on_Maps.pdf