I adore Frozen, but this announcement is disappointing, to say the least. If the Anna/Elsa M&G wait times and merchandise sales are any indication, a fully realized Arendelle mini-land with a big-ticket dark ride in a thematically appropriate location (e.g. New Fantasyland) would be popular. Instead, they're giving up on the Epcot theme and using the most quick and dirty way to get Frozen into the parks. That solution strikes me as less than satisfying for both Epcot fans and Frozen fans.
The Three Caballeros is a little bit of a thematic stretch for World Showcase, but since the film itself is basically a Latin America travelogue I've grown to accept its placement, though the execution leaves something to be desired. However, Frozen is so far removed from the original Snow Queen story that it really doesn't belong in World Showcase. And, seeing as how Gran Fiesta Tour is basically "hide-and-seek in Mexico with Donald", I doubt riders will get a Norwegian cultural experience on the new Frozen ride. In fact, I would wager that this ride will be less relevant to the host country than Gran Fiesta was.
I agree with
@RSoxNo1 about the placement of Frozen. DHS really needs family attractions, but there is just something off-putting about placing a fairy tale kingdom there, even if they are trying to get away from the working studio aspect. In my opinion, they should expand Pixar's presence in the back of DHS. I don't think they should group these properties together simply because they were made by the same studio, but most Pixar movies have more contemporary settings that don't really fit in MK.
This is another reason why I don't like putting Frozen in Epcot. It doesn't really solve any issues that WDW has. DHS needs more to do for families, and MK is still stuffed to the gills even after New Fantasyland. Meanwhile, Maelstrom and Norway are really the least of Disney's problems in World Showcase; yet that is where they are focusing their attention - in a way, I reiterate, that is underwhelming for both Epcot fans and Frozen fans.