I tell you my experience. I'm leveling a priest that it's 60 now and since I have also done some instances (and I have full heirloom set with 20% experience gain bonuses) and been very ahead of the guide, I skipped some zones since they were grey quest for me, well it was a real mess. Missing quests because I didn't to the previous to enable the following. Going in a zone then realizing that I couldn't do that quest because I didn't do the starting quest chain, very unconmfortable. In short what I suggest you it's not to skip areas, or make really sure that you do all the quests required (at least) and check carefully the previous zone you'll going to skip to make sure that you won't have problems with struggling finding where to start the quest chain to sync with the guide section.
Quite possibly the greatest source of information in regards to leveling I have seen for a new expansion. Bravo. Me and my friend are having a week long lan party at my house so we plan on leveling our mains to 120, doing all the week 1 shit and then starting at least another toon to get 120 as an alt (I personally am an altoholic, its bad, send help)
I defintely think at 118 you should go and do all four of these parts. These are required to unlock World Quests and probably take about 60-90 minutes to get through. If you do these whilst levelling (especially all in one go at 118) then when you hit max level you're ready to go on WQs. You can go finish the zones quests at your leisure over the next weeks/months or never even finish them if you don't care about questing.
Sargeras - Alliance Sargeras - Horde Scarshield Legion - Alli... Scarshield Legion - Horde Sen'jin - Alliance Sen'jin - Horde Shadowsong - Alliance Shadowsong - Horde Shattered Halls - Alliance Shattered Halls - Horde Shattered Hand - Alliance Shattered Hand - Horde Shattrath - Alliance Shattrath - Horde Shen'dralar - Alliance Shen'dralar - Horde Silvermoon - Alliance Silvermoon - Horde Sinstralis - Alliance Sinstralis - Horde Skullcrusher - Alliance Skullcrusher - Horde Spinebreaker - Alliance Spinebreaker - Horde Sporeggar - Alliance Sporeggar - Horde Steamwheedle Cartel - Al... Steamwheedle Cartel - Horde Stormrage - Alliance Stormrage - Horde Stormreaver - Alliance Stormreaver - Horde Stormscale - Alliance Stormscale - Horde Sunstrider - Alliance Sunstrider - Horde Suramar - Alliance Suramar - Horde Sylvanas - Alliance Sylvanas - Horde Sanguino - Alliance Sanguino - Horde Saurfang - Alliance Saurfang - Horde Soulflayer - Alliance Soulflayer - Horde
Something to keep in mind as well is that this will make enchanting materials more valuable, since it is likely most players will now simply scrap their boe greens instead of getting them disenchanted. Also redundant dungeon/raid gear can be scrapped rather than traded to the (guild) enchanter in the run. Less stuff gets disenchanted, meaning there will be less ecnhanting materials, meaning their value will rise. All the more reason to DE rather than scrap as an ecnhanter.