I am rather new to origami myself (also working through some models of Montroll) and I see several aspects where the difficulty in origami lies in:
- exact folding
It comes with practice. In the beginning I was getting bad results often, because inaccurate folds ended up messing things up several steps later. It still happens when I try new models, but it's not that much of a problem anymore.
- folding techniques
Again comes with practice. Some folds are intrinsically more difficult to execute than others. It also depends on the number of paper layers and the kind of paper involved.
As you seem to not have any introductory material explaining the basic folds I suggest you take a look at this page:
http://members.tripod.com/~PeterBudai/O ... lds_en.htm
- paper
Different papers are different to fold. Some allow for wet folding, some are stronger, others weaker, some allow easy reversal of valley/mountain folds, others don't, etc etc
- instructions/diagrams
As you have seen for yourself some instructions are very clear, while others leave you in a confused state. Experience helps to some point there. This very forum has also been a great resource when I ran into trouble interpreting diagrams.
- crease patterns
A big increase in difficulty comes from folding crease patterns. With no more detailed instructions a lot of guess-work and experimentation may be needed. I've only looked into CPs very briefly so far and am having huge troubles to get started on them. Even the most basic models appear hard to fold. As I've found out some CPs only appeared hard to me because I wasn't familiar with the folding sequence. After having done such a sequence due to diagrams of other models it is also easier to apply them to folding CPs.
- designing
I am totally unqualified on this topic, but assume that the design of your own models is a whole different level of origami again. It probably comes with its own set of more or less difficult problems.
Generally, I believe, that you can teach proper folding, basic folding techniques, and the handling of different kinds of paper to just about everyone. The application of this knowledge to the folding of models from diagrams is something most people will be able to (of course it needs lots of practice). I assume that the crease patterns are a big step up from there, such that there are probably many origami fans who will never reach the point of being able to fold models from a CP without exterior help. And I think an even smaller percentage of origami fans will be able to create good origami designs.
