1. Who owns the content on the site/service
2. How content you upload or create on the service/site can be used
3. What the service provider/site owner can do with your information
4. What the rules are for using the service/site
Of course the Terms of Service also cover much more, but those are the big pieces that affect most users. As someone who does not care for a video of my grandmother's birthday that's on YouTube and is "private" to be used in an ad campaign, I have made it a point to actually read what it is I am agreeing to before clicking accept. Although I can't promise that I will read the entirety of every single agreement top to bottom, I will skim and pick out those four points and see if it's something I can live with.
While you may have friends who are quick to complain about how certain sites work, have you ever asked if they read the Terms they agreed to before using the site? It's important to look at what exactly you're agreeing to before signing up, whether this is for your local softball team or an internet service, and we wonder why most people read the fine print for something in the "real world" but skip reading when it's something on their tech.
For tips on how to read those Terms quickly, check out this helpful post from Lifehacker.
Do you read the Terms of Service? Why or why not?