You did say role playing but not which setting :)
ARMA I/II:
Completely open world, with the ability to freely explore whenever you choose (between quests of course).
In this case between missions. You can move freely on the map, but don't be disappointed if you just cannot run into an enemy base without being captured or killed.
Usually it is best to choose your terrain and approach wisely, before running off.
Plenty of customization (even if just mostly cosmetic)
Totally customizable, play current time against terrorists or zombies, WW2 (extra game based on this engine exists), Vietnam, you name it...
Plenty of items to discover and use
Any weapon or weapon mods, vehicles, planes, helicopters, tanks, ships, bicycles, motorcycles you can imagine.
Quests than can change based on your actions and words.
The main plot has probably 3-4 ways to finish, join the enemy for example. Within missions you have completely free hand.
Multiple possible builds/play styles.
You are a human and there is no character sheet. Your eye/hand coordination and tactic defines your skill-level, but you do get better weapons/equipment from dead enemies or find them and therefore increase your abilities.
Unique in either it's art style/art direction or world design
This is no jump around FPS shooter (there are such multiplayer maps, though), mostly it is tactical in nature. You also get to command groups and larger troops on foot or armored/airborne while actively participating in the fights.
Or play it as MMORPG:
http://dayzmod.com/
Not too easy (no "press this button to win" type of games) and not too hard (no "you misused one skill point, therefore your build is now entirely useless" games)
You can make it as hard or realistic as you like or as easy as you like. There is a steep learning curve involved to play this game.
You get hit by something, you are injured and probably bleed. So you use a bandage to stop the bleeding. You cry out load because of pain and the NPC react to this, no sneaking any more. You should try to find or call for a medic!
You get shot badly, you roll over the ground and cry out load in pain. You die quickly without help, maybe one of your NPC team members will drag you to the side and provides first aid.
You get proper shot, you are dead and start from last save.
You tank or plane is destroyed and you made it out. You are probably injured and what remained of your crew? You could try to capture an enemy tank or plane or fight on as ground troop etc.
To sum it up, you either get sucked into this game or not. The community content is the best part of it. The mission replay-ability is very high, due to the randomness of the outcome and you can always adjust the difficulty level or the level of realism.