I can't speak for every maker but I suspect that it often depends on the leather more than the design itself. And that it also varies from maker to maker.
Generally speaking, however, it is not the style nor design specifically that I find difficult, it is the details. And, most importantly, the way the details add up to create the whole--the gestalt. This speaks to the second part of your question but also to the whole issue of quality and finesse and excellence. And it encompasses so much ...both visible and invisible...that it's hard to quantify.
Consumers either refine their eye or they don't. If they do, they notice "wobbles" in stitching, for instance, or broguing that is not evenly spaced or perpendicular to the line of stitching. All the superficial stuff...but stuff that is visible.
Most don't concern themselves with the invisible things--the construction techniques, choice of materials, etc.. But those are the very things that a good maker obsesses about....along with the superficial stuff.
And some of those things...esp. the superficial stuff...have been discussed in the opening posts of this thread.
Going back to styles...there are some styles that are ostensibly more difficult than others, of course. But again, the leather is a factor as well as the skill and experience that the maker brings to the table. Sometimes a difficult style can be pulled off but it begs the question if it should have even been attempted simply because the leather wasn't suitable for that style. Almost any whole cut (much less but esp, seamless wholecut)...from oxfords to pull on boots presents difficulties (and limitations) that are out of the ordinary. I've seen seamless ankle boots, for instance, that I admired (maybe even gobsmacked by) on one level and, on another level, had deep reservations about simply because the leather looked stressed.
I've seen shoes that were in your face with technique, such as gosier stitching, for example, which simply looked clumsy and crude to my eyes. Yet it is hard to find fault in a shoe such as this one--one of the finest shoes I've ever seen posted here, impeccable workmanship from top to bottom:
edited for punctuation and clarity
Edited by DWFII - 3/15/16 at 6:54am