Frozen isn't entirely original, but that doesn't make it less fun.
Having said that, "Frozen" is clearly based on the classic fairy-tale "The Snow Queen", by Hans Christian Andersen (1844). Peter Del Vecho, the director of "Frozen", said so himself. BUT - and it's a big but - it's still a fun adaptation, so that's good. There's nothing wrong with taking classic fairy-tales (or anything else, for that matter!) and adapting them to create something new.
Of course, taking the source material wholesale and claiming that you wrote it is completely different. But that's not at all what Disney did with "Frozen".
Then again, Frozen et al. came out years and years ago. I'm thinking of this latest trend - i.e. Disney taking its own animated films and CGI-fying the crap out of it. They can do what they like with their own films; I just would've liked to see them continue to make originals like Frozen, Tangled, Moana etc.