Before I get into this, I just want to say that you do not have to follow each and every word I say in this to make your custom map good. I'm just basing this off of community opinion and what I've seen people react to. Also, I may come back and update this every now and then when I get more ideas.

This is mainly for mappers who really want to improve on their maps and make them better and more memorable.

DO have some inspiration
Inspiration in my eyes is key when it comes to making a really good map. For example, my upcoming map Night Club has a scary and creepy feel to it. This was inspired by the first couple of Zombie maps, Nacht and Verruckt, because they had lots of creepy sounds and stuff that contributed to the horror feel. Of course, your map can have a different inspiration. Like ZK's Prison Mission being inspired by The Walking Dead. Whatever your inspiration is, put it to use, but of course, don't over do it.

DON'T make your inspiration the main focus
Sometimes maps that have been inspired by a movie or TV show or any video game kind of overdo the inspiration. They try to center the map all around the inspiration, and sometimes it even makes you forget that you're playing Zombies. So set some boundaries.

DO have lots of new features
Features are what gives the map an objective, or just add to the fun factor. However, these can make or break a map. Nevertheless, if you want to make your map fun by adding in some objectives, then add some that will fit with the map. You can script some yourself if you have that type of experience, but if you're new, there are countless amounts of scripts and downloadable features provided by other members of the community. However, there are some features that will render your map "unplayable" for some.

DON'T add unnecessary jump scares
We've all played Bridge and we were spooked by the random jump scares (not here to hate), and these kept some players from playing the map. This goes to show that you should only add a jump scare to a map if it fits. If you want to make a map that's creepy and scary, and you just want to have a little jump scare in there, then do it. Unnecessary jump scares are only when it doesn't even fit with the map at all. If your map is bright and sunny, then don't add any jump scares. Also, don't constantly spam jump scares behind every single door of the map. If anything, learn from Rise V2. That map did jump scares correctly and centered them around gullibility.

DO add lots of detail
Detail, above all, is the most CRUCIAL thing when it comes to custom zombie maps. There are tons of tutorials that show you how to work with terrain patching to make grassy hills or dirt piles or whatever you want with a brush. Add lots of xmodels to your map to contribute to your map's theme. If you have CoD4 for PC, download the mod tools for CoD4 and use Tom-BMX's CoD4 Xmodel Mover to add some models from CoD4 to give your map a modern feel. Also you can use Lime to rip models from MW2, Black Ops, MW3, Black Ops 2, Ghosts etc for you to use in your map. However, don't take detailing too far.

DON'T over detail
Never overfill your map with xmodels to the point that it makes it overly difficult to find things. Hobo Ville, for example, made finding the parts to building the power unbelievably difficult.

DON'T overly replicate Black Ops 2
In my opinion, Black Ops 2 completely did everything wrong. One thing that still bugs me to this day is that everyone hated the fact that you had to BUILD the power in TranZit, yet there are map makers that add buildable power to there maps, and they even said themselves that buildable power was a dumb idea. Never do what Black Ops 2 did wrong. Here's a small list if needed

DO have beta testers
Beta testers are the only way to know if your map is complete and ready for release. Make sure that you pick people who will actually find stuff in your map that is broken, glitched out, causes a script error, or overall just breaks the map. Most of the time "beta testers" will just play your map, and complement you on how good you did (trust me I did that once and the map I did it for didn't get positive community feedback). Anyways if the people you picked did not try to find anything broken in the map and just played it to see how you did, I'd suggest dropping them and not letting them beta test again. But if your testers did find broken stuff, FIX IT . Don't just let the stuff that is broken remain in the map just to be pointed out by the community after the map is released. And if you see the lines

If you find any bugs in the map, let me know please

in the release post, then the mapper either didn't have beta testers, or gave the map to people who said they would test the map for bugs, but just played the map and didn't look for anything at all.

DON'T give the "it's my first map so" excuse
This excuse aggravates me like it's nobody's business. In my eyes, it doesn't matter if it's your first map. At this point of time, custom zombies is nearly 5+ years old. We have seen maps that have heavy amounts of detail in them. I don't see how someone could make a less than detailed map as there first map. Never make that excuse. If it's your first map, take the constructive criticism into consideration and pay attention to what people liked, and what people didn't like. 

More to come soon possibly.

Hopefully this helps some of you guys out when you either make your next map or your first map. Also, I didn't make this to offend anyone. If you feel offended somehow, then don't post about it. There is nothing that should offend anyone at all on this.

Few points I would like to fill on with my opinions

DO have lots of new features

Some people dont have inspiration on making new features and there are a lot of people, that have done the features that you might come up, so its almost impossible to add new features, since they might have been in map before.
Besides, its nothing bad to re-use some custom features multiple times

DO add lots of detail

Again to the point of inspiration, some people dont have it and I would say that if the map is not broken, has some cool stuff and shows thought put into it, I myself would like the map. Detail is always good, but some people cant do it right away like some people

DON'T overly replicate Black Ops 2

If the map is a remake or oriented by BO2, I would like to see BO2 features instead of same old 3 teddys to open door to ending. Of course newer features are always wanted, but again, if people want to increase the difficulty, then let them add those features

DO have beta testers

Both maps that I have released didnt have beta testers, there was just me or two other people testing and this can be seen. I agree with this point, do have them, but sometimes you cant, like in contest, didnt want to risk it being leaked

DON'T give the "it's my first map so" excuse

This is true, dont give that excuse, you can still make good map with mediocre features cos of all downloads there is in net, even when they are overused. Releasing box map shows how much mapper cares more about getting their name everywhere than actually spending time on map. Rome wasnt build on day, nor are good maps made in a day

Good post, but some points are more of your opinions than communitys, havent seen that many people saying Dont make your inspiration main focus or Dont overly replicate BO2. Not going to insult anyone, but these are my opinions on these points

DON'T add unnecessary jump scares
We've all played Bridge and we were spooked by the random jump scares (not here to hate),...

then to love? come on, spread the hate!

:3 this is nice, tho I would like to see some features from bo2 in my opinion for maps (except buildable power)