One of the new systems Legion brought to the fore of World of Warcraft is level scaling.
Players are essentially allowed to choose any order of zones to quest in The Broken Isles. Blizzard not only made it clear this system would stay for future expansion Battle For Azeroth, but latest patch 7.3.5 brings it to expansions gone by.
Before this newest patch, players had predominantly levelled up from 15 through dungeons for years. Once Legion came along, they dungeoned to 100 then quested to 110 in The Broken Isles. The primary reason for ‘dungeoning’ through so much of World of Warcraft stems on it being significantly faster than traditional questing through zones. For a few years now, a lot of Azeroth has gone un-quested, and abandoned. Levelling became a task of whether you could be bothered to spam your way through dungeons for a couple of days, and in some cases, doing the same one five or six times in a row. I for one haven’t levelled recently because I’d gotten tired of that repetitive process.
There were other reasons to avoid levelling traditionally throughout a continent. Before this patch, players (with heirlooms especially) could level their way past a zone before finishing its story. Before 7.3.5, zones had very specific suggested levels - usually in increments of five. Some could be 23-27, and you’d get there, reach level 27, but you’d barely started the zone’s story. You'd generally get better experience by moving on to the next zone, so you’d move on, abandoning every quest in the process.
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Now, most zones in vanilla continents, Eastern Kingdoms and Kalimdor still have a suggested starting level (some much lower) and all scale to 60. Westfall is fine to quest between levels 10 to 60, for example. But this way it’s much harder for players to out-level a zone, and can now stay for as many levels as it takes to finish the story without losing out on gaining experience.
Level scaling effectively turns old content into new content by making levelling more rewarding and giving players more freedom. For more seasoned players like myself (10 years next month) we can quest in zones we remember enjoying, and visit areas we probably haven’t been to in years.
There are instances where you don’t have to do expansions chronologically, either; you can now quest in Northrend before going to Outlands, for instance. It may even be possible to skip Pandaria — that's what we all really wanted, right? Dungeons are still a much faster way to level up, however, but with Outlands and Northrend doable at the same time, you can do either expansion's dungeons making it easier to go to individual dungeons for their respective quests. Basically, it’s less likely you'll have to grind through the same dungeon multiple times in order to quickly level up.
Personally though, I don't care what's faster; I intend on levelling in World of Warcraft like the good old days, revisiting content I haven’t seen in many years. I’m looking forward to levelling with friends, picking zones we actually want to do rather than tediously doing Dire Maul six times in a row. For newer players who’ve only ever known levelling through dungeons, some of these zones will be completely new to them. This is surely the dream for developers: being able to breathe new life into old, forgotten content without completely reworking it.