I have been working on my own M(mini)MORPG (or ORPG) on and off for about a year now, though in truth it's the last 6 months where I have written most of the code. The only thing I would say to someone else looking to take on a project like this is do not underestimate the amount of work involved
I would tell you to stay away and work on some smaller projects first but that would make me a hypocrite as I dived straight in, though I have been a Java Developer by trade for a number of years now so I guess I have experience on my side.
As a heads up, you will run into issues with threading when trying to instantiate entities coming in through your connection thread client side, I heavily suggest using TWL as a backbone as methods such as GUI.invokeLater() give you instant access to the Slick processing thread and TWL's input handling is excellent.
If you fancy checking out my project to see how far I am, the link is in my sig.
Best of luck