On the basis that I haven't done the route (but have read the reports)
The section that most people seem to dread is from Lochinver via Glen Canisp to Ledmore Junction, seems to be: (1) ride for ten metres, (2) pick up bike and carry over boulders for ten metres, (3) go to step 1.
Other sections that "get a mention" are: the approach to Bealach Horn from Glen Golly (traversing peat hags); the climb up from Inverlael over to Dundonnel; the track from Poolewe to Loch Maree; the top of the climb out of Torridon; the climb out of Glen Lichd to get to Glen Affric.
Of course there are other technical sections as well
If I was going to do one part of the route then I'd choose the northern bit, mainly because I've not ridden any of it. I've walked in to Carnmore which is in the Fisherfield Forest and it's an amazing bit of country (fifth shot in the series here
http://www.highlandtrail.net/). Maybe start somewhere like Dingwall and do the upper bit though that does include most of the bad bits I've mentioned above
