Firstly, I'd just like to point out that we're still in the early stages of this xpack and things are subject to change, so I'll reserve my final judgement for patch 7.3. The artifact catch up in 7.1.5 is already going to help a bit (but won't fix the issue).
Here's a little quote from blizzard:
Underlined the two relevant parts. Blizzard has said on multiple occacions that they wanted to lower the gate to get into pvp and make it more accessible with these changes they made for Legion. Of course blizzard has said this before as well. However, the ending result is far from what they had hoped. Pvp is now in fact more gated and hidden behind higher walls than it has ever been in this decade. This has resulted into extremely low pvp participation rate.The only contribution your gear will make to your overall power is through a small modifier based on your average item level. For every point that your average item level increases, your pre-determined PvP stats will increase by 0.1%. That means a 25 item level difference between two players only results in a 2.5% difference in stats, compared to the 25% difference it makes today. There’s still a little incentive to improve your gear – a concept we think is important for World of Warcraft – but the benefits are much less pronounced.
These changes bring a couple of major advantages. First of all, it puts everyone participating in organized PvP on a much more even playing field. Obviously, you'll still want to unlock your Honor Talents to reach your full potential, but you'll be able to hold your own in battle in the meantime.
In the past, we had honor gear and conquest gear. This was gear tailored towards pvp, by adding resilience or pvp power, depending on what xpack we are talking about here. In Wod and Mop it was really easy to obtain this gear, thanks to the conquest catch up mechanic introduced in Mop. If you were behind in the season, no worries just keep playing and you can catch up. In Wod, thanks to Ashran, you could gear your fresh character for pvp within days of semi-casual ashran fragment farming. In addition to full honor gear you also got a few conquest pieces too. After you were done with that, you could just hit up with someone from the lfg tool and do some 2vs2 for the rest. It only took a few weeks tops to be fully geared for pvp with 710 gear. This gear also scaled up to 740 when you entered pvp in the open world or wherever, so the gear was the optimal choise for pvp all around. It was far from the best for pve, so it didn't interfere with anyones raiding process. This is the way it should have stayed in my honest opinion, blizz really hit the nail with that system. No clue why it had to change. The only downside to Wod pvp gearing was that you couldn't buy accolade trinkets with conquest, but that was a minor issue.
TODAY:
In order to get into semi-serious/serious pvp you now need:
1) Gear. Yes, gear. It still matters, a lot. Too bad gearing is pretty slow and the shittiest part is that you can't really do this via pvp now. You gotta get it from doing pve. Since all gear is the same, the best pvp gear also comes from pve no matter what the circumstances are, which is really a big fail from blizzard. I personally like the overall rng chance for gearing in legion (titanforge etc), but for pvp you need the predictability. Right now you can pvp all day long but get no progress in the gear deparment, so you're essentially just wasting your time (cept if you are prestiging). My suggestion is that they reintroduce pvp gear that's obtainable only from pvp. This gear could only have an ilevel of 855 or 860 or whatever, but it needs to jump up higher when you enter a pvp instance or engage in pvp outside in the world, just like in Wod. Alternatively they could make gear a complete non-issue in pvp and just go with the stat templates that won't changed. You could still earn gear from pvp, however.
2) Artifact power. This is actually a bigger issue than gear as the passive damage and health increases apply in pvp as well. In 7.1.5 it'll be easier to gain ap with your alts, but reaching the max level with your artifact will still take tremendous amount of playing time. This will maintain the gate high and thus deter people away from pvp as people with maxed out artifacts will smash you. Pvp is a decent way to earn ap, but personally I'd like to at least see the passive damage and health increases go away in pvp.
3) Pvp talents. People have been suggesting these for ages, and now we get them. I like the idea of them, but they way they are implemented is far from optimal. Why do you need to grind them, again? This part is rather agonizing, really. Sure, if you're really into pvp it won't take long to unlock them, but if you rather do pve/quests for your gear, it'll take a while (specially if you like alts). This is another unneccesary gate they've created. The talents are needed in instanced pvp in order to succeed, but they aren't powerful enough to make a difference in the open world and thusly they don't distinguish "serious pvp'ers" from pve'ers enough if you are being ganked. Personally, they should just give the talents to everyone without a need to grind them when you enter a pvp fight anywhere.
So tldr; you need gear, artifact power and pvp talents now in order to do pvp. This is a huge contrast to Mop and Wod, which really had an extremely low gate to get into pvp. In legion blizzard wanted to lower the gate to get into pvp, but instead they created a higher gate than ever before in this decade.
People are saying the pvp participation rate is now at an all time low. I don't know if this is true or not (probably is), but it's really a no brainer why that is. It's the poor way of gearing through pvp combined with the extremely high gate to get into pvp without the feeling of being smashed. Now, I'm sure someone is going to start creating foam in their mouth right now and scream "BUT YOU PVP FOR FUN NOT GEAR". That may be true for you, but for most peope, this is a mmorpg not an fps. You need the sense of progression. The whole point of mmorpgs is developing your character and making it stronger. If pvp won't offer this sensation, it'll deter many people. Even pvp needs to offer carrots to those that participate in it.
The low participation rate has nothing to do with balance however. Class balance has always been whack and it will keep fluctuating one way or the other, always. Sometimes melees are strong, sometimes casters will be stronger. This'll be tweaked over the course of the xpack like always. For example, TBC had extremely high arena participation rate even though class balance was abysmal. Only a handful of specs were even viable. Ladders were active thanks to the carrots being so good, aka you could earn epics that were almost as good as raiding gear even for pve. S5 in wotlk was also really active even though it was completely melee dominated with abysmal one shot fest style class balance.
Pvp is not really more dumbed down than it has before either. I'm sure that is something that would matter to 2,5k+ players, but for the large masses it doesn't matter. There's no point whining about dumbing down if you are stuck at 1800 rating. In TBC and Wotlk you had less buttons to press than you do now for most specs.
The prestige system is a good addition, though, and will help to keep battlegrounds active.