First off, if you didn’t already know, links that consist of nothing more than “Click here” or “Learn more” are generally frowned upon. How do I know what you are linking to is of any worth to me when you don’t describe what I’ll see on the next page? Granted, one could use the context clues of the information surrounding the link to assume where the site is trying to take you next. But why would you make your users think when you could have just supplied a better description in the link itself?
Generally, I try to come up with a link label that describes the subject of a link in isolation. If you were to remove everything else from the page, would you be able to determine where you would arrive after clicking on the link? If so, you’ve got good link text.
I think this probably comes into play with users who rely on some kind of assistive technology, including but not limited to screen reader users. Imagine this: you navigate to a page using nothing but a screen reader and a keyboard. First thing you do might be to check the heading outline to gauge what’s on the page. That might not be enough information, so you have the screenreader read back a list of links you included on the page, say a blog post. A bunch of “Click here” links or straight URLs isn’t going to give you any more indication as to what you’ll find in the rest of the text of the page.
Anyway, this is my criteria for writing better link text: make sure the link makes sense on its own with all the context of the surrounding page removed.