Some points have been mentioned already, but I'll mention them again anyway!
1) Community - If you got a good community, then you're off to a good start! WOO! A good community is what I and many other potential players look for first when looking for a new RO server. (or any game for that matter)
2) Rates - I'm a Mid-Rate player myself, so I like my rates to be around 50-100x with a drop rate that's consistent with the exp rate. (25-75x) If you have unusual base xp & job rates that don't go well together, this may put players off from joining.
3) Max Level - This goes hand in hand with the rates. Personally as a Mid-Rate player, I like my max level to be 99/70. Depending on how high your rates are, you'll want a max level that's easily attained without too much effort, since most players play HRs for the easy/fast levelling.
4) Donations - Any server that heavily favours donators will often put players off from joining. If you intend to have a donation system, make them fair & not overpowered.
5) End-game - By this, I mean does your server have something to do when players reach max level? (aside the usual pvp/mvp) Any unique max level quests? Any custom features? Any scripted events that occur every hour or so? Things like this may attract more players.
6) Decent GM team - Another important one, make sure your GM team is mature enough to handle their roles. No one wants to join a server where GMs sit in towns spamming commands to gain attention. I joined a server last month where the GM was randomly killing people in town cause he was bored. I uninstalled 2 minutes later.

7) NPCs - Being the more casual/lazy player that I am, I like to have several custom NPCs handy, such as a warper, healer, job changer.. etc. On a high rate, a lack of these type of NPCs would probably put players off from joining.

WoE - Try to spread out WoE times if possible, so various players around the world can participate. This may be something to consider as your server grows more. Also, it's a good idea to have a limited number of castles open relative to your server's population. If your current population is around 20, then it'd be best to just have 1 castle open for now.
9) Limited customs - Custom features/items are always nice to have, but don't overdo it. Don't add/change so much that it becomes a completely different game, otherwise you may struggle to increase your population.
(not saying that it's a bad idea, but I've seen many heavily customised servers struggle to gain a high population)These are just general points that myself and others often consider when looking for a server.
Good luck.