The only thing that worries me about the internet's role in 40k is the effect it has on new players, particularly young ones.
Kids today (I can't believe I just said that...) spend a hell of a lot of time online and so they are going to quite reasonably assume that everyone who plays 40k is also on a message board talking about it, like they are. I've abandoned a few forums over the years because they wold start to develop "styles" of list or tactic particular to that forum as a result of more respected/high posting members believing it was the best list. Anyone who disagrees was told (sometimes very bluntly) that they were wrong by the popular member's lackeys, or the member themselves. This is not a good thing. New players can easily be influenced by these little fashions and misled into thinking they have to buy certain models or they will have zero chance of ever winning a game.
Say for example you're a 14 year old just getting into the hobby. You visit a forum to get a feel for it, you read everyone loudly saying guard is a strong army and all guard armies need multiple vets with meltas in a transport. So you go and buy exactly that. Where was your choice? Do you even like Guard? Even if you liked the look of Eldar better, you now have Guard because you believed the hype.
I guess my point is statements like this damage the hobby:
"Tau are fail"
"Put 3 meltas in a chimera, rinse and repeat."
"Only an idiot would field ogryns."
Just because something is a popular viewpoint doesn't mean it's correct, and I think that's the biggest danger of the net. Individual thought gets swamped by fashion.
Sorry for the long post, I've thought about this a lot.