I suppose the best way of measuring the curve is how long it takes me to teach new site build clients, and therefore by definition new Wordpress users, how to be able to do all the basics, up to and including tweaking some code.

Time varies according to commitment but as far as 'difficulty' goes - at least 5 times easier in the last 5 years - in fact 5 years ago I wouldn't have attempted it if the client had zero techno knowledge. Now I enjoy doing what I do best which is the design aspect and I happily hand over the reins to the new owner when it comes to everything else!