This topic feels incredibly tangental to me. I think GND hit the nail on the head and most of the counters to his argument are really avoiding arguing his actual point, and instead trying to hit on extraneous stuff that isn't relevant. Things GND did not say:
Tau units aren't overcosted
Tau units received fair compensation for the loss of JSJ
Tau are more mobile than they used to be
Yet the vast majority of the posts in this thread are strictly about how removing JSJ wasn't fair because Tau aren't as good anymore. The argument wasn't that removing JSJ made Tau better, it was that JSJ was not a good mechanic to have and we should have other mechanics to replace it that are more interesting.
JSJ is simply not a good mechanic from a design perspective. Warhammer is a game of interaction; nobody is here to play solitaire. And yet that is exactly what JSJ supports and emphasizes. Especially in an army that focuses primarily on shooting. It doesn't let your opponent get to play the game. Quite frankly people play units to do the things they're good at. Nobody is picking a melee army because they find it really fun chasing people around the table. They want combat rounds. They want to feel like they're getting an opportunity to engage the opponent in a meaningful way.
Additionally, it's not a mechanic that requires a true choice on the player's end. It is purely beneficial. Sure, there may be times where you're already perfectly positioned, but you didn't choose not to use JSJ because there was a potential negative side effect, there was just no positive benefit. Warhammer is a game that requires no mechanical skill (you can't practice throwing dice better, or being better at moving a piece accurately when there's no pressure of time), so the fun of the game ultimately comes down to the decisions we get to make. JSJ did give you a choice of where to move, but it was almost always extremely obvious (move where you are in the least danger). As everyone has pointed out, Tau aren't exactly superstars in melee. There was no tradeoff of wanting to be in close combat. Simply put, the best location to be was mathematically possible to calculate. This does not feel like a decision; it is simply whether or not you know the answer.
A good decision in Warhammer comes down to deciding whether or not you think the risk of an action is worth the reward. This was never a facet of JSJ as there was no risk, simply only reward relative to how well you knew how to position.
And if you needed another reason to dislike JSJ, I would point out that it unfairly prioritizes many attributes and makes it difficult to balance Tau units and give us true variety and choice in army building. The mechanic heavily de-emphasizes units that have strong defensive stats, as the idea was to make our units survivable by not taking any damage to begin with. It also shuns relevancy in melee combat and decreases the value of having range on your weapons because you could safely deal damage with much shorter range ones. All of these things are aspects of suits in 8th edition that can be played up to give us more variety and options when creating lists. It allows GW to diversify unit characteristics without them feeling weak.
GW recognized that shooting was still the style of the Tau, so they gave us the option to leave combat freely with many of our units. In this way, our opponent gets to meaningfully interact with our units, and we still get to focus on ranged damage. As many have already mentioned, there likely needs to be more in the way of survivability as a reward, but ultimately this is a far more fun and interesting way to play the game. As long as we still have legitimate avenues of winning, it creates a friendlier and more engaging environment. Ultimately, we all want an opportunity to feel like we had a chance to win the game. And while you could argue that players certainly had more opportunities than they may have realized in 7th edition, the removal of JSJ has given more players the ability to feel like they're having a good time when playing against Tau.
Let's not discredit GW so easily. The index obviously has been unkind to many of us in terms of balance, but that frustration is not a good reason to restore a poor mechanic. If you look at the index changes, it's hard to expect them to have been incredibly well balanced at the first go. Tau was an army that received incredibly wide, drastic changes. There was no reasonable amount of testing that could have been done internally to get a good idea of just how good they felt. It was more about changing the direction than it was about getting things super even. Too much focus on pure balance was impractical, as armies are simply given more options as there codexes are released. Even if they spent a whole lot of time on it, it would never have fully compensated for the additional strategems, artifacts, and just raw game examples for balancing that the last several months have given them. What they did gain from the index was an idea that they preferred the way Tau interacted with other players and other armies in 8th, and that's awesome. Now we just have to be patient and wait for the index to come in and balance things.Getting things taken away from you sucks, there is always a sense of missing it when it's gone. But what it seems like GND was really getting at was that it's an emotional response. When you take a step back, calm down, and think about whether or not JSJ was really good for the game, you'll probably realize that there's a lot more fun stuff that could be done instead. In an ironic way, Warhammer is actually a game where there are no rules. There is no rule that can't be broken, no idea that can't come to fruition through the game mechanics. Let's use our imaginations and not just go back to something old just because it's familiar.