Lots of things, but some off the top of my head:

Communication technologies probably top the list. Sure, the Internet has given birth to lots of great communities, like the one where I'm typing this comment. But it has also created a hugely polarized environment. (See the picture on page 4 of this study.) It's ever easier to follow your biases and only read the opinions of people who agree with you, and to think that anyone who disagrees is stupid or evil or both. On one hand, it's great that people can withdraw to their own subcultures where they feel comfortable, but the groupthink that this allows...

"Television is the first truly democratic culture - the first culture available to everybody and entirely governed by what the people want. The most terrifying thing is what people do want." -- Clive Barnes. That's even more true for the Internet.

Also, it's getting easier and easier to work, study and live for weeks without talking to anyone else than the grocery store clerk. I don't think that's a particularly good thing from a mental health perspective.