Surnames were a really cool idea in theory, but the way they were implemented was horribly, horribly wrong.
What's wrong about it, you ask? The fact that players are forced to use them, that's what.
Back in 2010, Hiromichi Tanaka said that surnames were implemented to ensure that players could always get the name they wanted. Well guess what? The name I want is just one name: Rhianu. But the game didn't allow me to have just this name, instead forcing me to tack-on a second name after it. The surname system did not ensure I would get the name I wanted. In fact, it did exactly the opposite. It deprived me of my desired name.
I do not want to have two names for my character. I want to have only one name. Forcing me to have two names takes away my freedom of choice and makes it impossible for me to create the online persona I truly want.
Not to mention the fact that surnames also make everyone's names extremely long, which is annoying because it fills more screen space and makes it take longer to type out someone's name when sending a /tell.
Also, for some odd reason I've found that it also makes it harder to recognize people when everyone has such long names. Not sure why that is, but it just is.
I understand that surnames do carry with them certain advantages, such as being able to get any first name you want even if it's already taken, as well as enabling an extra layer of creativity and cleverness that simply isn't possible otherwise, but these advantages are offset by the disadvantages I've listed above. But if surnames were optional, all the advantages they entail could still be gained without any of the disadvantages that come with forcing them.