I'll come out and say that I think that Legion is one of the best expansions World of Warcraft has ever had. After the supreme disappointment of Warlords of Draenor, Legion has felt not only like a return to form, but an expansion that has contained a kind of ambition that I wasn't sure Blizzard still had for the game. Legion competes with Wrath in my mind for best expansion (and of course there are plenty of system changes over the years that have solidly improved things over time, such that if we were simply dumped into Wrath as it was in 2008, I probably wouldn't like things as much mechanically.)
One of the interesting changes to come in Legion was the expansion of the dual-spec idea. Rather than having too "modes" for your character - something that was primarily useful for those of us who like to do group content as a tank or healer but prefer to solo as DPS - they now let us change between all three specs any time we're outside of combat. It really equates to the old idea of "tri-spec" (or quad spec for Druids.)
So on the surface, this means that switching specs is easier than it ever has been in Legion. And to be fair, if you're playing a character who is below level 100, that's true.
But once you get into the current expansion's content, you get somewhat locked in.
Two factors are at play here:
The first is Artifact Weapons. This is particularly noticeable for classes like Mages or Warlocks, who could use identical gear for all three specs. Now, if you want to switch specs, you need to really invest a lot of time in leveling up a different artifact weapon.
They do provide a kind of built-in catch up. Between the steady progress of artifact knowledge and the fact that traits become more expensive on an exponential rate, it will probably take you far less time to get twenty traits in a second artifact unlocked than it did to get them in your first.
Still, especially now that we're getting to a point where some players have their full 54 traits unlocked (that's including the 20 "prestige" traits that I think Blizzard expected almost no one to complete,) it means a really massive investment of time and effort into fully powering-up your artifact weapon.
And with no more artifact knowledge coming at least until 7.2, we can't expect it to be easy to catch up on these prestige traits.
The second factor is legendary gear.
Legendaries in Legion are a very different beast than they've been in prior expansions. In fact, these pieces of gear now work almost exactly as they do in Diablo III - each has a special effect that often interacts with your class' specific spells or abilities.
In Diablo III, once you get to the level cap, you basically try to get a full set of legendary items (along with an armor set - the set bonuses in Diablo are far, far more powerful, increasing damage done by like 20,000% depending on your build.) In Legion, you can get some very powerful legendaries, but the droprates on these are such that you're really not expecting to have access to all of them.
But because the droprate has bad-luck protection - which, glass half-full side, means that you're more likely to get one if you haven't yet but glass half-empty side means you're less likely to get one if you already have one - it means that getting a legendary for one spec means you're less likely to get one for a different spec. For example, my Mage just got his first legendary while doing an LFR wing in Frost spec after having played as Arcane for much of the expansion. The good news is that I think 7.1.5 fixed most of the problems I had with Frost this expansion, so I'm ok with playing that spec again. But for example, my Demon Hunter has been lucky enough to get two legendaries, both of which are for the Vegeance spec. Nowadays he's primarily Havoc, meaning that the only benefit he gets from those pieces are the admittedly great stats.
Ultimately, these might not be such a bad thing. Back before 3.1, you chose a spec and you stuck with it. Changing spec (or even just redistributing your talents) was a pretty costly endeavor, costing up to 50 gold back when that was a significant chunk of change. We lived with that system - and that was before they really made it easy to solo in any spec.
Still, dual-spec was one of the most popular features they ever added to the game, so you could argue that reverting to that "choose a spec and stick with it" system isn't really a good move.
I'll be curious to see how things work out going forward. They say we'll be leaving our artifacts behind in the next expansion (but keeping the appearances for transmog, I believe) and I imagine that they'll come up with a different system for legendaries (perhaps going back to the Mists/Warlords model.) So maybe these issues will simply evaporate.