What makes a map good is highly subjective. Astro crater has a very simple layout, does not require expanding all around the map, can run at an okay speed on weaker computers, etc. So, some people prefer to play on Astro crater and consider it a good map, while some others think it is an abomination. Many players really enjoy playing on Dual Gap, but many hate it. The same could be said for many other maps. The perspective on what makes a map good can also vary a lot between groups like high-rated ladder players, wonder/hilly/canis/mapgen/etc team game players, Setons players, etc.
So, for example, the matchmaker team has its own general set of standards that it uses when evaluating maps, and even then, things like player rating/skill and team size significantly come into play when judging if a map is good for X. Additionally, there are undoubtedly lessons to learn about what makes a good map (for a lot of players' standards) from maps like Dual Gap, Setons, and Astro that have had such lasting popularity, even if some of those lessons strongly conflict with the standards of the matchmaker team/mapping veterans/ladder pros/etc.
There are many spectrums that contribute to players' opinions about maps; turtle to snowball, clustered mex distribution to spread out mex distribution, low mex to high mex, low reclaim to high reclaim, flat to mountainous, simple to complex, detailed aesthetics to simple aesthetics that require less processing power, large to small, low player count to high player count, no water to entirely water, etc.
While there are strong merits to many of the pros' standards, I have found it funny how big of a discrepancy there is between what most of the 'mapping/ladder pros' think makes a good map and what most players actually want out of a map. Specifically, a lot of players like more eco-heavy/clustered spawn/simple map design with more relaxed gameplay that focuses less on t1 spam-based map control than what the 'pros' generally prefer.
Despite that, I have written a mapping checklist that covers a lot of what makes a map good for a lot of people in a way that is theoretically generally compatible with the pros' standards. However, even it would still exclude a lot of popular gameplay. So, make of that what you will, but here is the checklist:
proper slope indication (where is passable vs impassable and buildable vs unbuildable is clearly communicated)
no technical problems (slots are odd vs even, proper naming/versioning is used, the map/code works properly, there are no broken mexes/hydros, the map folder is <30 mb, and the map is not too laggy)
legal (work is not stolen, not used without permission if permission is required, and not used without attribution if attribution is required)
proper texturing (textures don't showcase repeating patterns too obviously, are sufficiently blended, have proper albedos/normals/scales, have sufficient variety, and fit a sufficiently cohesive theme)
cohesive aesthetics (terrain/texture/decal/prop/marker/unit/lighting/water/skybox usage and placement work cohesively to form an overall map that doesn't make any of its components look too out of place)
tolerable aesthetics (map is not too hard on the eyes, not too unpleasant to look at, and doesn't have any sufficiently significant aesthetic mistakes (such as excessive floating trees))
balanced (the map contains no cheats, intends fair balance, does not too heavily favor any factions, and does not confer a much greater level of asymmetric imbalance than Seton’s Clutch does)
quantity and placement of mexes/hydros/reclaim supports a variety of each of the following being reasonably viable: strategies, unit compositions, locations of battle, layers of battle (air, surface navy, submersible, land armies, long-range indirect fire), tech levels of battle
usage of different slopes/terrain features/water supports a variety of each of the following being reasonably viable: strategies, unit compositions, locations of battle, layers of battle (air, surface navy, submersible, land armies, long-range indirect fire), tech levels of battle
PS: I think moreso in terms of percentages/ratios of mexes rather than concrete mex counts, for what is safe vs raidable vs contested.
