You can be arrested for things just by using Tor if they mix you up with someone else or something?
(Also check the new relay guide: https://trac.torproject.org/projects/tor/wiki/TorRelayGuide)
But the sad truth is that there aren't that many hosting providers that allow Tor relays. Especially exit relays, because of abuse complaints.
Also, as you might expect, Tor relays can use lots of bandwidth. It's more common to get flat-rate bandwidth for 100 Mbps uplinks, and metered bandwidth for 1 Gbps uplinks. Digital Ocean, for example, just switched to metered bandwidth, and that has killed some relays.
However, all this could arguably change, if Tor became mainstream, as part of Firefox.
Run an exit node and then do something illegal. Then blame it on someone else.
So running an exit at home to coverup for posting on a forum that bans all Tor exits, that makes no sense.
If you use tor and visit the regular web sites (like, say, HN), the last computer that does the actual request to the website is an exit node, as far as that site is concerned, the exit node made the http request. If you run an exit node, your computer is going to be doing tons of requests to all kinds of websites, this may include sites that deal in illegal stuff like drugs, child prostitution, human trafficking, terrorism, etc.
edit: Forgot to say, you must explicit be running an exit node. Not every tor node is an exit node.