You might not run into any problems yourself, but on the other hand you might hit every performance killer, maintenance nightmare, non-portable feature and broken concept there is!
Trust me, I know how that goes. It's not game development, but I write Java web-apps for a living. (everything from simple servlets to full-on EJBs) Java is an amazing, incredible, stupendous language that just makes you want to beat the living crap out of something sometimes. It's aggravating how much you have to "look up" to work in Java. In C++, you can't do something, you just brute force it. Start writing directly to memory and make it work the way you want. In Java, you have to find somebody's API and learn how to use it . . . ugh. Just figuring out AWT, BufferStrategies, MouseListeners, VolatileImages blah blah blah has been a pain. I think the worst of it is over for the moment though. Who knows . . . I may come to eat those words.
The most common mistake I see is people picking projects beyond their abilitys, or letting features creep into a simple design until it's not viable anymore.
Oh don't worry . . . I've already done that.

For example. Everybody starts out with identical races which evolve based on their playing style at key points in the game. So far I've come up with 18 different races to "evolve" into and I have two more "evolution" points to design yet. *sigh* May have to pare that down a bit.
Not to mention Incredible tech tree, micromanagement-reducing automation . . . customizable to each individual player's preferences . . . and did I mention the fully customizable GUI?
Nah . . . that's not ambitious.

it's just when I hear "real-time strategy" or "multiplayer" and "RPG" when I want to stab myself in the face repeatedly.
Oh . . . so now's not the time to say that my game is just practice for developing my multiplayer real-time strategy/RPG game?
What? How hard could it be?
the two monsters in this field are Ascendancy and Master of Orion 2
Played MOO2 and wasn't terribly . . . wowed or anything . . though it was a good game. Never played Ascendancy. The one I played a LOT was Stars! A great little game when it was made, though definitely a bit old by now . . . and the micromanagement was just insane in big games. Mare Crisium is the company trying to develop the sequel. (company's founder wrote the original) It looks sharp (not in Java tho) and will be a great game . . . . if they can ever get out of "we have no money" hell and back into production. They got as far as beta testing and have been stuck there for 2 years now. Having a publisher helps. *sigh* ah well
Well, thanks for the reality check . . . time to get back to work, I guess.