I navigate quite a lot, and for years I've seen all these annoyances that I always think we should all punish their authors by leaving instantly the page.

You should leave the site when...

It modified the size of your browser.
They want to catch your attention, and as the content isn't good enough that's the last resource they have. And the excuse that they want to make sure the user has the browser big enough is just not valid.

It takes ages to load
And they weren't clever enough to make the loading time entertaining. Websites are supposed to be for more than 1 visit. If your loading takes ages, people will have the experience just once. I think you are missing the point.

You need to register before entering the site.
No way! That is so 1999. If you want to catch users/emails, be creative and find a way so the people wants to know more about you/the product and invite them to subscribe at some part of the experience, but if you make then register first thing you'll get a lot of a@a.com, if any.

The page is all full of ADs.
Specially in blogs. Good content creates money/opportunities in the end. If you have ADs around from the start you're creating a money pressure to yourself that is likely distort the quality of your content.

You have to watch a unrelated AD first.
That is very common on flash games. You have to see that AD before playing (supposedly while it loads). I find it very annoying and it makes the game lose a lot of its value and character. As in the previous point, your games are designed to earn money to yourself and not to provide a good gaming experience, which is what should be.

Am I missing any?

I could put a example of every case, but they don't deserve the publicity ;) Unfortunately, I don't think this will change anything, but somebody had to expose it.