The last big patch for Dungeons and Dragons Online added a new feat for spellcasters: Augment Summoning. It adds +4 to the stats of everything you summon or charm, as well as +4 to hireling stats. With this relatively minor addition to the game, Turbine essentially created a new character specialization. The feat is a lot butcher at low levels than at high levels, but at low levels it is absolutely amazing. I discovered this by accident on a low level character I recently started.
On a low level wizard, a common strategy is to charm a foe in the middle of a big crowd. The guy you charm will start attacking, and gain aggro from the entire crowd. That leaves you free to come in behind the melee and beat down your opponents one by one. However, if you charm a minion you can't generally expect them to help much. They are mainly a distraction.
Not so now with augment summoning, at least at low levels. With augment summoning, a charmed target hulks out and becomes a killing machine. A random charmed minion in a crowd will sometimes murder everyone around them before you can even get within melee range. It's sort of like the scene from a horror movie (this seems to happen in anime the most often) where one of the good guys suddenly gets possessed/ infected / whatever and starts killing everyone before they die in a hail of gunfire. I'm having a lot of fun with it, but I have to wonder how long before it gets nerfed.
More generally, to me it's a good example of how something that doesn't sound like a big deal on paper can be a major change to a game. Being a summoning/ charming specialist is now suddenly a very attractive character specialization, especially for solo work. Turbine added a new development path for casters with a single feat. Specialized charms for foes such as undead and slimes I would not have really considered slotting before suddenly are a lot more attractive. Role Playing Games (MMO or otherwise), seemingly more than any other game genre, can be greatly altered by changes that would be equivalent to small tweaks in other games.