He's the original, and thus he has his flaws. Take reviews he's done, and compare them to the works of other reviewers. A perfect example would be his Street Fighter 2010 review, also done by SpoonyOne of The Spoony Experiment and TGWTG. While both are enjoyable videos, Spooney's is much more in depth, as well as he beats the game. AVGN just sort of talks about Street Fighter, then brings up 2010, talks about it, end of review.
He follows an older retro reviewer style, the style in which the game is handled much like a child would. A kid in the eighties wouldn't have the internet, forums, gamefaqs, youtube walktroughs. The Nerd often only works with a few issues of Nintendo Power. Its kind of cool in that way. Makes it feel like an adult reliving his traumatic experiences in the dark ages of games, when reviews were hard found, games were bought with luck involved on if it was good, and a cryptic challenge remained that way until you and a few friends shared info on the playground.
As for immature he does do a few too many toilet jokes. And he does use swears alot. But it adds to the style.