Anyway, I'm going Mount and Blade Warband. Ya it's not open world in the sense that you see your character walking everywhere, but you do move around the map in an open world and getting into a battle will throw you into an open area that fits the location of the map.
The game probably has the best melee combat I've ever played. You have to move your mouse in the direction you want to attack (and block if you're not using auto-block -which you shouldn't if you ever want to be competent at multiplayer). Momentum is hugely important when it comes to landing a decent blow. Also, the type of weapon you use has a significant impact on how you might play. Taking a pike will mean you want to be at the front line, ready to annihilate the cavalry charge (if the AI chooses to use one). If you use a huge two-handed weapon, you can expect to often one shot guys, but you'll need to constantly be moving for fear of getting mowed down by archers. If you go sword and shield, you can expect to well in sieges since you'll be able to block the constant hail of arrows. That's just a handful of the foot soldier scenarios. There's still mounted combat too!
Anway, the combat system definitely takes some getting used to since it'll probably be unlike anything you've ever played. However, once you get the hang of it, you'll find it exhilarating, challenging, and quite strategic. Then, once you've mastered the system to the point where you can easily take on multiple enemies at a time, jump online and get absolutely shit on. It's there that the depth of the system truly shines. Still, it's great in single player too if that's all you're looking for. I spent 300 hours on this game (a lot of this is with some amazing mods) before even touching multiplayer.
The graphics may turn you off, though, to be fair, you don't really notice when you're leading a 50 man cavalry charge into the enemy's flank, trampling and skewering half of their army. For me, it's pretty much the greatest feeling in gaming.