I think one of the biggest things people are worried about is the decline of the English language in its ability to effectively describe things, events, feelings, abstract concepts, etc. We don't use 'thee' anymore, in the future I think 'gonna' will be a proper word, and (sadly), perhaps even 'could of' will become accepted use.
I think the worry is that the overall quality of the teaching of English is declining, so that in the next few generations people won't have the vocabulary to describe things as well as we used to. We take the easy way out, we use spell-checkers and grammar checkers, or we just expect (correctly) that other people will understand because it's 'just' an online conversation, and it'd be ridiculous to assume that people should put so much effort into communicating properly. But I think the real danger is that if we rely so much on shortcuts, we'll gradually lose our ability to describe and talk poetically, with great detail and with romance and with humor and wit and everything that makes the 'classic' novel a 'classic'.
"Man I feel like shit my gf is a bitch and the other day we got into a fight about how she wanted to see her friends and she said I'm holding her back what do i do?!???"

"
Language change and evolution is good, but I think we have to try (becase it's hard!) to distinguish what is being <b>a)</b> left out and dropped along the way because it's archaic, and we then have ways of replacing that, and <b>b)</b> what is being left out because we're lazy, and don't want to really put effort into our speech. Our ways of communicating will be negatively affected, and on the extreme end we'll end up communicating in grunts again! Well, obviously we won't go that far, but I do worry (because I'm weird) that we won't be able to speak with the eloquence and effectiveness and the efficiency that we could in the past. (And I realize the irony, considering my essay-size responses on the TFP!)
The problem is that when such a large amount of your typing is online communication, and the shorthand is used, the likelihood of that dominating your speech, I think, is pretty high. I don't think the SMS-speak (c u l8r m8, r u cumin 2 da show tnite?, etc) will overwhelm us, but things like 'he did it good', 'gonna', 'could of', 'definately', etc. will probably come into common usage.