What do you think about world building toward gameplay?
I think it's the best version of worldbuilding. I see absolutely no point in wasting time on elements of the world the players will never encounter. Is it possible they will encounter something soon? Then start making a rough sketch of that thing. Is it unlikely that they will ever encounter something? Then don't waste time on it.
What are the techniques a world builder can use to build toward gameplay, and what should be avoided?
Don't prep anything that's not interactive. The best example is the Great Wheel and the various planes that instantly kill the PCs. Avoid that. Better to create something like the Elemental Chaos of 4E. That's playable. I would seriously suggest anyone interested in worldbuilding with an eye towards playability take a long, hard look at 4E's cosmology and points of light setting, and Sly Flourish's Return of the Lazy Dungeon Master.
What happens when a world builder finds they are building more for their own pleasure or for bespoke stories than for gameplay?
The referee tends to lore dump on the players and push the game towards revealing their worldbuilding.
How can they salvage their world?
Yes. As per the common writing advice, "kill your darlings." Anything not directly in front of the PCs doesn't matter. Remove the deep lore. Nothing is true until it appears in game. Set the binders aside and focus on the next session. The broad strokes will survive, probably. The details and minutia will not survive. Be okay with that.
What TTRPG worlds really speak to you from a "built for gameplay" perspective, and which are interesting worlds but fail in the gameplay department?
D&D 4E's points of light setting and the cosmology they built for 4E was literally designed from the ground up with gameplay in mind.
Eberron straddles the line for me. It was originally built for gameplay. The setting is dripping with inspiration and plot hooks. But after almost 20 years there's so much built up and so much of the map filled in that it can be really hard to play in that setting without either ignoring decades of stuff or being boxed in by the lore. But...Eberron also mostly sidesteps that issue by having the start date fixed across editions allowing the referee to move ahead in time and change things as they go. Winding up a world on the brink of war and seeing where it goes based on the factions involved and the PCs' actions is really amazingly fun.
Settings like Middle Earth are the other end of the spectrum. So utterly locked down and filled in that there's effectively no potential left for new adventures.
You need blank spots on the map and in the lore for the PCs to explore for there to be a game to play.
To me, the ultimate in "gameplay first" would be a starting village and a few nearby hexes with points of interest to explore.