Only two trends really annoy me:

"Meh". I know this didn't start on the 'Net, but it's never grown on me, though I've tried.

The "bad grammar joke" trend. This is kind of hard to summarize, but it started on sites like icanhascheezburger.com and has steadily infected informal chatboards everywhere, with pictures and phrases like "I paralyzes you" and "I has a flavor", and on and on. It was funny once, but it's been run into the ground, IMO.

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"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

--The State