These are guidelines that should be followed when making goomods. Most of these are absolutely necessary and failure to follow them can result in your goomod not being published.

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1.

Read the documentation! Please, before you start making goomods it is essential that you read the reference guide about making levels. It tells you about how levels work, which can help you make much better and intuitive levels.
Also, it's highly recommended that you read about Addin File Formats and Game File Formats. These will give you a better understanding of how goomods and World of Goo work, which is necessary in making more complex addins. There are also various other pages around the site that have valuable information, these and the pages mentioned above can all be found under the Developers tab (at the top of the site).
It may be a lot to read, but trust me, it pays off. And you don't have to memorize all at once, you can always come back and check up on bits and pieces.

2.

Choose the correct section. Although this is fairly obvious, I'd just like to mention that you should take care when choosing which section to submit your addin in. For example don't upload a goomod that changes World of Goo Corporation balls in the "Ball Styles (Other)" section.
With some addins it can be difficult to choose sections, and in that case mods can move them to the correct section, but most of the time you should know where it goes.

3.

Don't give goomods abstract file names. Your goomod must be given a unique file name that somehow indicates what the mod is. The preferred format is:

com.goofans.[username].[mod name]

Names such as "mylevel" or "newBalloon" will not be accepted. These same rules apply to goomod IDs. Ideally the file name and ID should be the same.

Also, including a plus (+) in the file name, such as "com.goofans.puggsoy+AP.greatbun" will make the goomod unable to be downloaded, so don't do that. An alternative would be to use a hyphen (-) instead: "com.goofans.puggsoy-AP.greatbun".

(Note that this rule is about file names. The actual name of your goomod can be anything you like, as long as it's related to the level.)

4.

Give descriptive descriptions. That is, on your addin page, write a description that tells us something that we should know about your goomod. If it uses some special feature, if it represents anything, stuff like that. People want to know about the addin they're going to install. Descriptions like "this is my first level hope you like it :)" are OK, and will be accepted, but most people won't really see a reason to download it.
If there are any special notes regarding technical details, please include those too. Things like if the goomod is incompatible with others, or if it should be installed by itself, are important. Also if there are multiple goomods on the download page that do different things you MUST point this out.
Also, you should upload a screenshot with your level. Usually a preview of your addin is preferred if only to lighten up the download page. If you don't know how to make a screenshot, see this page on taking addin screenshots. Although this isn't absolutely 100% necessary, most addins without a screenshot won't be published.

5.

Do not submit uninteresting goomods. Tests, clones of original levels, or generally anything that doesn't add something new and interesting to the game will not be published. People install addins to expand World of Goo, not to see what you created in 5 minutes.
This also counts for gooballs: new gooballs must have a purpose. If you want to submit a gooball that only looks different, submit it as a World of Goo Corporation Ball Style.
Daft as Brush wrote in detail what sort of goomods you should and shouldn't make. Although it's quite old and some points may be slightly outdated, most of it still counts, so I recommend giving it a read.

6.

Think about the players. When making an addin, think about how other people will experience it. If you make a level, it will seem easy to you: you made it and know exactly how to complete it. However, other people might have extreme difficulty completing it. When testing your level, imagine not having seen it before and how you would go about finishing it. Try and see how others will see your addin, not only yourself.
If it's your intention to make a near-impossible addin (such as PeterB's Whack Pack), then please state so clearly in the description. Unless you say so, we will assume that your level should be completable by the average WoG player, not a master.

7.

When making chapters, know what you're doing. Study the site's notes on island.xml, look at others' chapters, try to find out as much about them as possible. Experimenting with the chapter-related files yourself is a good place to see how they are put together.
Basically, I'm trying to say don't go delving into chapter making if you don't know about it. Although it looks simple, it's much more complicated than making levels and at the current time there's no proper tutorial for it. So until such a time comes, be prepared and take your time with getting your chapters to work.

8.

Be careful when editing Chapter 1. It is highly, highly recommended that you do NOT otouch Chapter 1 unless it is ABSOLUTELY ESSENTIAL to your addin. Chapter 2 is fine, any other chapters are fine, but please stay away from editing Chapter 1 if possible.
The reason for this is simple: Chapter 1 is where GooTool installs level addins. You know, they go in the top right corner. Editing Chapter 1 could effect this, which will make either the level, your chapter, or both, unplayable.
Now, if it is 100% necessary to edit Chapter 1 in your addin (such as with 5-chapter total conversions) please edit it through merging. This will greatly decrease the risk of level addins stuffing up and increase the chance that people will be able to install them along with your addin.
If, however, your chapter absolutely cannot be merged, you must point out CLEARLY in your description that no other addins can be installed at the same time as yours. You must state this as obviously as possible, so that only blind people won't notice it.
Please read over this rule again. I must stress that it you should NOT attempt to make a Chapter 1 editing goomod unless it is strongly needed by your addin.

9.

When releasing level packs don't make multiple goomods to on the same download page. Most people will click on the "Install Now" button, which only downloads the newest goomod, and they'll miss out on the rest.
If you want to upload a level pack, merge your levels into one goomod by following MOM4Evr's goomod merging tutorial. If you want you can also just upload the levels as separate addins on separate download pages, but it won't really be a "pack" then.

10.

Moderators are people too. Please please please, be patient when awaiting your addins to be published. We moderators are real people with real lives, and we are doing this completely voluntarily. We have no obligation whatsoever to publish or test addins, we are doing it purely out of free will. Sometimes there are also lots of addins to test too, and it can take a long time for us to get round to it.
Now, I'd like to mention a few things about how the publishing goes. You see, there is a queue for addins to be published. Whenever someone requests that an addin be published (by clicking the "request to be published" button), an email is sent to every moderator, and it is added to the bottom of the queue. We usually test addins from top to bottom, so addins that are requested first will probably be tested first. Once we've tested the addin, we will either publish it, in which case you will receive an email stating that, or we will deny it, in which case you'll also get an email saying that we denied it. If we do deny it, we'll comment on the addin saying why it was denied. Once you've fixed the problem you can request publication again.
Now, if you get impatient and request a second time before it has been denied, that's not good. First of all, it sends us another email saying that an addin wants to be publsihed while we already know that it wants to be published, and that just annoys us and wastes our email space. Second of all, your addin will get sent to the bottom of the queue, so it will take longer for us to publish it. So please, do not request publication a second time unless you have fixed a problem with your denied addin.

11.

Do NOT change an addin's ID when updating it and uploading a new version. In fact, don't change it ever. GooTool keeps track of which addins are installed by their ID, and if you change that, all hell breaks loose. Terrible things will happen when people install your addin and they won't have a clue why.
So when updating your addin, only change the version number. Never, EVER change the ID. Ever.

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Thank you for reading these guidelines. If you manage to follow these then I'm sure you'll make better addins, they'll get published quicker, and more people will want to play them.

Happy goomodding, and goo-d luck!