The biggest threat to JJP IMO is Hobbit fatigue... I did not think the first movie was all that great after a single viewing.
The biggest flaw compared to Fellowship of the Ring? The dwarves were not really characters. Having a floppy hat or being rotund does not magically make you a character that I should care about. Multiply that by ~13, and you have so much extra baggage wandering around that I was honestly hoping they would lose most of them so they could actually bother to develop a few of them.
Compared to Fellowship of the ring where the core characters are quickly fleshed out and different... and you actually care about who's who. Even Bilbo comes across as being a bit of a jerk at times in the film, and I assume he's supposed to be the hero
So yes it was pretty and yes it had throwbacks to the LoTR movies that fans like me loved... but it was a bit of a fail in terms of keeping me interested, I can't imagine what someone who had never seen LoTR would think (maybe they would like it more, who knows... maybe character development is for suckers ).
Where was I going with this? If the next few movies follow down this path, you could easily get Star Wars Prequel Syndrome, where the first movie is a big deal, but by the time Episode III rolled around general interest had waned and you are left with mostly hardcore fans. So the broad appeal of this theme might not be as broad as you would think based upon the success of LoTR.
Still two movies to go, but if somehow the next movie is worse (like Attack of the Clones, IMO) all bets are off in terms of Hobbit preorders