Anyone with $450 in RF gear can hijack the “smart TVs” in their area. Sending out a malicious “app,” this hijacker would be able to cause thousands- or tens of thousands of network-connected TVs to do just about anything.
(The case example — all the TVs in one neighborhood spamming Yelp with good reviews for a local restaurant.)
Of course this is more of a concern when seen from the perspective of a targeted attack. Anyone can attack YOUR TV, and (if you plugged in the Cat-5) use it as a backdoor into your network, or hijack your accounts if you surf the web on said TV.
Even if you’re smart enough not to connect your TV to the Internet — wait, if you’re that smart, why do you still watch TV? — an attacker can still mess with it in an arbitrary fashion.