Diamond was the latest game I played, so I can't speak from too much personal experience. I do have some thoughts from reading through this thread, which I thought I'd share.
It seems hard to deny that Pokémon has a major railroading problem; the developer's attempts to ensure that the player follows the general plot set up for them are often pretty painfully conspicuous, especially in hindsight. From the beginning, these included arbitrary roadblocks, whose removal was superficially coincidental with actions of players which actually were required to cause them, which have been apparently exacerbated since HMs have been done away with, now being left the only option.
The one advantage HMs had over these was that they allowed the player to personally clear the path, making it less arbitrary, and also involved the player's pokémon themselves. However, this pretty clearly came at a high cost - hidden moves were, excepting surf, generally pure utility moves with very situational uses, which would often restrict a player's party and their movesets to include them. These problems were clear nuisances to many, making their removal well worth the cost of their advantages.
So, it seems to me, going off both Aduro's and the other users' comments, there are a few positives held by HMs, but which probably can be returned to Pokémon without outright returning to HMs themselves. A few possible solutions I thought up:
- Doing away with (hard) roadblocks altogether. A lot of the roadblocks HMs were designed to solve seem to be dedicated to enforcing a certain order of gym battles. While having their own set of problems, HMs were at least openly made available as a result of earning badges, unlike the egregious arbitrary events which held you back until your unfinished business in the latest city was done. Perhaps a solution would be to do away with the moveslot element, but return to the badge-granted authority; have any attack be able to get rid of roadblocks, but have such terraforming be a right reserved to those who have earned the right badge. This might not be as easily applied to the likes of surf, fly and flash, but it's a thought.
- Doing away with (hard) roadblocks altogether. Of course, it is a question how necessary strictly enforcing such an order is. If further levels and battles get more difficult, and all badges are required to challenge the League anyway, it seems arguable that challenging the gyms in the order that they come is the most reasonable choice anyway. Perhaps the difficulty curve in itself is actually sufficient in at least nudging players towards linear playstyle, and straight-up enforcing it isn't needed - maybe ambitious players ought to be allowed to go the harder route of venturing into the more treacherous areas, and taking on the more difficult gyms right away, which might be too much for some, but never with permanent repercussions, what with there always being pokémon centres, and the option to backtrack and grind in easier areas.
Of course, that being said, another option might be a suggestion I've heard from a few people...
- Doing away with linear playstyle altogether. The fact that the gyms display a difficulty curve that just so happens to match the route the player's character takes to the league has always been a tad curious, and especially considering that the gym leaders display alternate teams when faced again implies that the gym leaders might in fact not have progressively stronger teams, so much as they present challenges which suit the number of badges their challenger has - which in most cases for the player is a foregone conclusion for each leader. What if that suggestion were made an open fact, and instead of being designed as a line of progressively higher-level teams, the gym leaders were each given eight different possible teams, the one they use referring to the challenger's badge count? The difficulty curve would still exist, but seem much less arbitrary, and make the region feel more natural, rather than a tailored progressive quest made by the developer. Of course, apart from the gyms, there are a few other game elements that might be harder to adapt out of linear playstyle. Wild encounters being higher level on "later" areas of the game is something that would be hard to adapt to this style, as wild pokémon wouldn't have such etiquette as trainers so as to not attack those with lower level pokémon. Though, then again, wild pokémon having a different level average in different places was always a bit odd. Evil teams would likely be even harder to incorporate into a game like this, though then again, maybe this will end up meaning players will have to go out of their way to find legendaries rather than have game antagonists inevitably leading them right to them once they've conveniently got their master ball. It's all just a thought, really.
In short, it seems doing away with HMs had benefits and costs, and the costs probably would be better dealt with by trying something new rather than reverting to HMs.