The main difficulty seems to be in the acknowledgement of gray areas - if I build a 1/72-scale USS Lexington (CV-2) and it takes me say 6 years to do it - to me it is still a model. If it is accurately built, scaled, painted and everything else - it may be more - it may be considered by some to be a replica and possibly even a work of art - but it is still a model, too. Obviously it can be more than one of these things at once - the same way an F-14 can be a fighter and a bomber. Take off, drop some bombs, shoot down some MiGs, and land. Fighter and bomber, all rolled into one.
I did the work, I built it (no matter what material I used), I painted it - all reliant on whatever knowledge and skills I possess. Toys like the GI Joe vehicles are mass-produced by machinery - I wouldn't call them models. Out of the factory they are identical. Model kits rely on the individual builder to give them life and an identity of their own, no matter how beautiful or hideous it may end up being. It is still my model. Nobody else did it, and nobody else is able to do what I did. It's not a competition - it's just the builder against themselves. Seriously - do you build a model strictly to win a competition? Is that why you do it? No - you build it because you want to see what you can make out of those parts that come out of the box - whether they be plastic, resin, wood...whatever.
There's a gray area where something like a diecast car - a replica, which is bought already finished, and involves no skill or knowledge on the part of the buyer, is on the low end, if you will. then there are things beyond a replica like that, where some skill or knowledge might be required in order to complete it. Still a replica, not a model? Possibly - if you go to the toy store and buy a R/C tank that runs and is complete OOB...that is a replica. If you have to build it, but afterwards it still does the same things as the OOB version - well, in between there is the gray area. How much individual work is required in order to qualify it for model status? It's an opinion thing, is what it boils down to. But that doesn't mean there can't be debate. Debate is fun.
But personally, I think a model requires a little of everything - building, painting - representing how you personally feel about the subject. It may not always end up looking the way you planned or dreamed it would - and that's part of the challenge. Maybe that is what makes it a model - the possibility or risk of failure.