Really? Unlike Lost, BSG and Twin Peaks, Carnivàle was pretty fully plotted from the get-go. Sure, the story isn't completely wrapped up, but I'd say that 80-90% of the questions posed in the first two seasons are answered by the show's end (check out the super-detailed Wiki pages if you don't already believe). What nagging mysteries eluded you?
Looks good to me. I wonder what happened to Charlie Jade, I just watched it on Netflix streaming service a few months ago. They have it for free on Hulu though.
To me, "cliffhanger" means that something big is just about to happen/get started. See: Marty stuck in 1955 again at the end of BttF II, "The Best of Both Worlds, Part I". But Carnivàle was always intended to be a six-season trilogy of three two-season parts, and, given the tone of the finale's last scenes, I expect there would have been a lot more build-up to the next big event in the overall story. So sure, the epic was unfinished, but I wouldn't call the finale a cliffhanger per se.
I'm pretty sceptical of the idea of defining cliffhanger in such a way you don't have to say an obvious cliffhanger is a cliffhanger, so I checked Wikipedia's brief. How Carnivale leaves us definitely qualifies, Spoiler: Carnivale ending with a main character going all black eyeballs on us. That's pretty clearly fitting the 'ensure the audience will return' bit, and also 'confronted with a shocking revelation' bit.