Defense of the Ancients, or simply DotA. I would put that Warcraft 3 custom map right next to Counter-Strike as one of the most influential mods ever.  People have finally started to pay attention to the fact that, hey, a competitive team-based experience with a mini-RPG experience inside of it is pretty awesome. Some of the original minds behind that map are developers at Riot Games, which has released League of Legends.
 
League of Legends is bringing it all a step further by also adding an MMO-lite level of persistence. Summoner levels, talent trees, and rune books. They all add tiny little bonuses that you can tweak and fiddle with to your heart's content. At the end of the day, if you are a good player, it won't change the game TOO much, but it would be a bit of a stretch to expect a level 1 fresh out the gate to beat a level 30 decked out person. 
 
That being said, the game is free to play. You gain an in-game currency (Influence Points) that can be used to purchase heroes to use and runes to put in your runebooks. If you feel so inclined, you can purchase Riot Points, which can be used for little things that aren't game changers, such as being able to purchase heroes early, or purchase 'boosts' to the rate in which you gain experience and influence points. Nothing game changing, just convenience items. 
 
Another big point of the formula is the fact that each team fights alongside an allied army. Waves of creatures that are friendly to you are struggling in battle against the other team's creatures. You are merely the champions, the heroes that will turn the tide of the war in your favor, and inevitably crush the opponent's base.
 
So - why should developers pay attention to what they are doing? Because it's frickin' smart. People have already warmed to the fact that they don't mind a little RPG in their games, and League of Legends combines both a match-based leveling system and an out-of-match leveling system flawlessly. Make the leveling and such within the game feel quite powerful, and make the out-of-match stuff be a much smaller level of power, but something that you'll start to feel a difference in after a while. Add on top of that the feeling of being in a larger conflict with the AI-controlled hordes, and it really takes it to a next level.
 
Those features, along with the robust item and item combination system ingame, makes for a type of gameplay that totally has plenty of room for expansion. 
 
Monday Night Combat is sort of doing the same thing, albeit without the out-of-match persistency, and that is in third-person shooter form, and in a more condensed format. A Modern Warfare-esque game could do quite well with the system, but swap out the in-match item system for the out-of-match weapon system from Modern Warfare 2. Leveling within a match would give you access to various ways to assist the soliders that are fighting on your side, trying to push in to take over an objective, such as a capture point or a defensive turret. 
  
Icefrog, the developer who was in charge of the Defense of the Ancients Allstars map for Warcraft 3 for quite a while, has been hired by Valve. That is quite an exciting prospect, since Valve is primarily a shooter company. If any company has been known to foster the innovation of smaller teams, it's Valve. Could a shooter with DotA-style attributes be on the horizon? Man, I sure friggin' hope so.