I wrote a mail which I did not send immediately, because I was still uploading something which was linked to in the mail.
When it came to send the mail, I opened it from Apple Mail's "drafts" folder, and... it just sat there, empty, thinking and thinking and thinking, trying to recreate the content of the mail.
I did something else, giving it time, and it finally succeeded after several minutes! It turned out that I was opening a temporarily created version of the email where I had mistakenly copied/pasted the name of a folder on my computer instead of a web link, and Mail, unlike my old email app, does not interpret this as simple a bit of text, it understands it as me copy/pasting the actual folder into the mail... And in this case it was a 600MB folder with thousands of files, all of which it apparently, bravely, was getting itself reading to email to my poor correspondent!
If I have to send a list of files to somebody, I have to first copy/paste the list into a text editor, which converts it into just text, and then copy/paste this list, otherwise I end up mailing the files instead of the list!
I guess it's useful once you know about it and get used to it. Though I still think simple drag/drop is the easiest way to add an attachment.