I've put in all the tags and everything but the upload button just produces a loading spinner on the tab with nothing changing on the page even after half an hour. Am I doing something wrong, is there a particularly short limit on video length for first time posters, etc?

Ah, if you put in the tags before uploading, you are in the wrong place. Video uploads have their own interface.

"Upload Video" is right next to "Upload", to the right.

Produces the same result unfortunately, but thanks, I'll give it another shot some other time!

There is also a size limit of a 100 MBs? that is, it should be just under 100 MB, even if by just one byte.

And keep in mind, for reasons beyond my comprehension, the server wastes CPU time to lossily re-compress every video uploaded to a fraction of its size, so expect it to get pretty shitty-looking unless you upload at the highest quality you can make it.