If you love running old content for transmog items, you'll love the new legacy loot system. Whenever a player enters a dungeon or raid where they are 10 or more levels above the content, the dungeon or raid bosses drop an amount of loot equivalent to what would have dropped for a full group when the content was current. That means players could earn loot for five people in dungeons and loot for 20 people after soloing a raid. The legacy loot system also drops other gear types, like cloth dropping for a plate wearer. It's now much easier to get the transmog gear you want, and you can also collect gear for your alts on the same run.
Groups are constantly forming in the group finder to go battle the enemy faction. If you’re more of a lone wolf, you can still stalk around the world looking for enemy players. You’re bound to find some. World quests with Warmode on also spark some seriously interesting interactions, especially ones for neutral factions like the Champions of Azeroth.
For Alliance it's Lok'Modan, Redridge, Darkshore, Felwood, Stranglethorn (both parts), Plaguelands. Somewhere between 3 - 3.5 of these get you to 60 in like a 16 hours /played, if you queue for dungeons at the same time. I recommend dropping everything at 45 and going to Darkshore-Felwood if you didn't do them yet - they are literally the fastest levelling zones in the game right now, maybe only trumped by Silverpine.
Update: The reason for less experience from gathering is due to how Blizzard revamped professions and broke them down by expansion. In order to get full exp from gathering you'll simply need to find a profession trainer for the relevant expansion. (WoD you just mine or herb the first one ypu come across and the game gives you a scroll for learning WoD gathering).
As late as possible. Early on, you’ll want to follow the missions in Battle for Azeroth right up until you get the option to send a follower on a two-hour long quest. As soon as you get there, stop doing the War Campaign missions. You can come back to them once you have reached something like 119 and a half and finish leveling with those if you want. If you don’t use these missions to cap off your leveling journey, then they’re the first thing you’ll want to do once you reach max level.
Players’ relationships with the NPCs around them have been hit-and-miss over the years. One of the original issues with Thrall, the Horde’s original Warchief, is that players began to refer to him as “green Jesus,” because of how infinitely powerful and infallible he was. But he’s been gone since the Warlord of Draenor expansion, and with the death of Varian Wrynn and Vol’Jin at the start of Legion, the old guard hasn’t been there to guide players the same way as it used to be.