Oh boy... possible rant time. o:
What I like about servers is most and foremost the community. Nice and helpful people who are fun to be around and play with. They are what ultimately makes a server a great server. On the other hand, if you have a bunch of immature and unruly members the server quality will drop immediately and no matter how nice it looks I would not play there.
While it's not overly important, how your server looks can mean many different things. If I can look at the spawn and see that tons of time and care has been put into it then I am pretty much certain that the owner really cares about how his server is presented and wants it to succeed. Most server owners that I've seen just stick a quick spawn together and call it a day. The way I see it, if a server owner has that kind of to his spawn then he will have the same attitude to the server. A lazy server owner is not a good server owner and the server of a lazy server owner is not a good server.
This brings me to my third and final point. Server management. A good owner knows how to manage a server. He is always listening to feedback and trying to make everyone happy (even if at times it's nearly impossible), He is able to come up with fun and creative ideas which will ultimately make his server stand out amongst the rest, and he is able to find mature staff to help him run the server. These are only a few properties of a good server owner. I can go on about owners who only make a server for the money or ones that are clearly unqualified for the position but then I'd simply be here all day (you could always read the blogs on my profile if you want more epic ranting).
All in all there are three things that make a server a great server:
-A mature and hard working owner (+staff)
-A server that looks good and you can see the hard work that has been put into it
-And most importantly, a great community
Hope that helps! Good luck with the server! :D