In my opinion, i think the most important thing that first and foremost have a server is balance. Whichever client you go, players mainly looking characters that can compete rivals equally well with other characters.
Then, i've noticed that lots of people really playing IL until today despite the years that have passed. This does not mean that everyone will play there.
Each client has his own peculiarity. Now, depending on what they want most (i'm talking about rates/enchants etc..), you have to do the right moves.
If you're gonna open a low rate server, you have to make it stable first of all and then, you need to make some unique/spesicif features of yours, that you think, that the players won't leave you server, in the first 3 months or earlier. If you are going to make a high/pvp rate server, you need to make some events, or some custom items which you'll not be able to find them so easily. Generally you should keep your interest the players active.
As i finish, i'ld like to say that it is important to note that corruption is a big part in this case that can be emphasized with some simple things. Like for example, you should have an experienced staff who will really cares for your server and they can prove this with daily operations. Also the combination of all the above shows that if you follow this recipe, if not sure, you gonna rock.
Do not forget to remind you that the advertise of the server is a very important. The more people will known one server, the better, because the more players are in the game, the more interesting it becomes.
I hope i make my self clear. HF.
-KeepDaFaith