The Final Episode of Teen Titans is Dark

Teen Titans does not end on a "cliffhanger."

A cliffhanger means you're left in suspense because the story wasn't resolved, and/or a new element was introduced setting up a new story that was never told (like Young Justice season 2, at least, until they revived the show). The story of Things Change comes to a close. It's all about Beast Boy living in the past and ignoring his friends and allies who need him in the present. The episode ends with him saying good-bye to Terra and going to help him friends. Sure, they don't show the rest of the battle with Whatever-the-hell-this-is-supposed-to-be (I actually like that it's not explained), but is anyone really left in suspense as to what happens? The Titans win. Of course they win. They beat Trigon the Terrible, do you really think they can't handle a random monster?

You could argue that the scene with Slade in the hall of mirrors suggests he's planning something and wants to keep Beast Boy from getting in his way, and they never tell the rest of that story, but that's not really introducing a new storyline. It's Slade. He's always planning something, and the Titans always stop him.

Think of Beast Boy running outside as a "hero rides off into the sunset" moment. It's like the ending of Justice League: Unlimited--we don't know what happened to Luthor and Darkseid, we don't know what happens with GL and Hawkgirl, and we don't see the fleeing super-villains get rounded up, but we see our heroes running off to their next adventure and we can fill in the blanks ourselves.

The finale of Teen Titans is abrupt, even a little anti-climactic (especially if you're focused on the White Monster B-plot), but there's not enough mystery as to what happens next for it to be a cliffhanger. If you're really starved for more Titans content, watch The Trouble in Tokyo or read Teen Titans Go! (that is, the comics that take place in the original show's continuity, not to be confused with the spoof cartoon of the same name).