Simple: List the most overrated games, concepts, characters, consoles etc, and then offer up an underrated counterpart:
Call of Duty Black Ops 2: Excellent game, but is it really worth the sales numbers?
Guild Wars 2: Again, an excellent game, but it just never really clicked with me and I don't understand why. It seemed remarkably shallow.
Graphics: I need to have great looking games, but I've seen too little improvement in the last 3-4 years for me to buy a game based solely on graphics quality. I think WoW still looks great and that is 8 years old with just a few improvements.
Warhammer 40K Space Marine: Not as good as CODBLOPS2, to be fair, but this game was somewhat overlooked, and captured the whole 40K universe beautifully.
Dragon Age 2: I never understood why so many people soured on this game. Sure the original was better, but this wasn't exactly the Godfather 3 of PC games.
Voiceover: It seems like such a throwaway sometimes, but if you have a few great voice actors who really sell a script, it makes such a difference.