This is a simple guide, as we, the mappers, aren't the all-knowing minds who can decipher generic stuff on first glance.

BAD FEEDBACK:

• "The map sucks" (Doesn't need any explanation)
• "It has bad item placement" (Provide examples, if possible, suggest solutions)
• "The map is badly designed" (See above)
• "It has the BFG/NG/CG/PL/RG" (especially when care has been taken in order to limit these weapons' damage with the design of the map itself)

GOOD FEEDBACK: (If possible, also suggest solutions)

• "Placing the Quad Damage near the BFG is bad for the balance of the map."
• "There are no safe places from the Railgun, it breaks the map as it is."
• "The Grenade Launcher's position favors spamming and it destroys the flow of the map."

CHECK LIST FOR MAPPING AND TESTING (May require a bit of help, suggestions are welcome)
(Also may help with design decisions)

IN GENERAL

• Can any player be visible at all times? (i.e. is the map well-lit)
• Is there any bug in the map?
  • Is it intended, unintended and will be fixed, or unintended but won't be fixed?
  • Are there items with "start in solid" or "not reachable for bots" in the console?
  • Can the bots navigate the map well in botmatches?
• How easy is the map to navigate?
  • How obstructive are the hazardous areas to the map's flow?
  • Can the player remember key areas of the map without having to resort to location names?
• How well distributed are the items across the map?
  • Is there a cache of powerful weapons and items somewhere in the map?
  • How worthy are the items near the hazardous area? And in the area itself?
• Are there spots for hiding? How easy is to locate them?
• How many chokepoints are in the map?

BASE-BASED TEAM GAMETYPES (CTF, One Flag CTF, Harvester, Overload)

• How easy is the map to navigate?
  • How easy is the enemy base/objective to find?
  • How easy are the bases to attack?
  • How easy are the bases to defend?
  • How long does it take to capture a flag in the shortest route without trickjumping?
  • How long does it take to capture a flag in the longest route without trickjumping?
  • Aside of the enemies, was there another obstacle to the capture which broke too much the flow of the map?
  • How many routes does the map have between the flags (not the bases)?
  • Are the chokepoints well protected from possible campers/snipers?
  • Is there any hazardless (including water) route protected from snipers?
  • Do the corridors contain any kind of cover?
• How easy is to make a flag capture? (CTF)
• How easy is to get the neutral objective and score at the enemy objective? (One Flag CTF/Harvester)
• How easy is to attack the enemy base? (CTF/Overload)

Am I missing something?

This is a really good overview on how folks should evaluate / rate / comment on maps. Unfortunately it's about 20 years late! :D

I think it's never too late to provide some useful instructions for newer users/players on how to properly give feedback, it's just like a guide on how to submit a map, or some other advice regarding etiquette 😀

Most of the listed points are even applicable to feedback on Quake Chsampions' maps. However, a custom QC map is technically way more complicated to make, as the game's source code is still not opened. Such a map is more likely to appear in the QC:DE mod for Classic Doom.