That's a great question. It's a mixture of technical skill and intellectual depth.

For example, throughout Scotland placenames are a mixture of old Gaelic, modern Gaelic and Norse, and then there are the Anglicised versions on top. So learning what placenames mean is something I did in school and it's a fascinating subject.

So when Gildor is talking with Frodo, and mentions the "...Branduin, that you call the Brandywine..." (quote from memory; might not be correct), that's exactly what happens with placenames. It's that level of depth (in all aspects, this is just an example) that makes it a great work. You can read more on just the name of a river at https://tolkiengateway.net/wiki/Baranduin

Enjoyment of art is, as you say, totally subjective, but anyone who's artistic can generally identify when something is a great work even if they don't personally like it.

Edit: if you like this kind of stuff, there are also some cool blog posts on the logistics of the Battle of Gonder: https://acoup.blog/2019/05/10/collections-the-siege-of-gondo...