DISCLAIMER: I am in no way telling you not to do any of these things. This is just advice.
1. The "Time Machine" Mechanic
I have seen far two many mods in which there is a time machine puzzle mechanic. (Traveling between clean and destroyed maps). I'm not saying something like that won't work. It's just highly overused.
2. Re-Creating Portal
I've seen far too many mods with the goal of re-creating the original Portal. Just take some time to come up with a basic outline of your Mod's story.
3. Going on Steam Greenlight early
I once saw a mod that... how should I put this... was made by someone inexperienced. And it was posted on steam greenlight. Valve and the community are going to only look for mods that have quality and effort. Heck, even I don't plan on posting Aperture:Evacuation on greenlight.
4. Lack or Improper use of detail
To go along with the previous statement, there is a huge difference between a bare wall, and a wall with cables and circuit breakers. Adding detail to your maps will make a huge difference in the level of quality and overall look. It is also important that you have decent knowledge of hammer.
It is also important to use detail in the right way. Don't use stuff from the modern facility in underground levels unless there's a valid reason. In my mod I'm sort of bending this rule, because the test chambers are from the late 80s. Another don't: using the yellow backpanel texture for everything.
5. Posting Mods without content.
I have seen this a bunch. Sometimes people post mods with no screenshots or content, just stating their "plans for the mod. You get the impression that there isn't even content at all.
I hope some of these have been helpful. If any fellow modders have things I missed, let me know in the comments.
Set a release date for 2 or 3 years later. Don't do that, when things are done they will be done.
Create only 1 mod per time. A max of 2, if they are connected somehow
Don't fix mods for other people unless the mods are really old or it have been broken by steam pipe updates, because of that not even the creators know how to fix it (as you can see on CWTH the mod is sooooo broken but the guy even released a steampipe fix. That would be an phenomenal mod if it worked)
Be very careful when modding portal 2 - Portal 2 modding is broken, a good tip is to only make models and maps, and avoid doing the portal 2 coding and only do it when you greenlight your mod so valve can make a template for your mod and you can do the rest of the stuff safely.
Don't post to moddb until you have at least 2 polished and very finished maps.
In case you wanna learn source engine and hammer, do it from the start.
Get a good computer before you ever start thinking to mod. Or at least before you think of recording a video. In case you wanna record a video go to a friend's house and use his computer
Some of these I learned from personal experience :/
Oh yeah don't forget to make content updates (like screenshoots and articles) like each 3 months or something, not only that give hope to people it also increase the entusiasm and it will be less likely to be a dead mod