When I want to read a book, I can just do that. When I play a game, I want it to have gameplay. Tworment makes you go through many hours of reading between moments of actually getting to play the game, and the reading isn't interesting. The setting fails because every square foot of it is screaming "hey look at how weird this thing is", and when everything is weird the weird becomes mundane. It doesn't feel like a world people live in, it feels like a creative writing major has cornered you and is now assaulting you with every idea he's ever come up with.
Most of that holds true for the PS:T too though, right?
So what makes Numera suck? World less interesting?
The writing is definitely of lesser quality and the world has a less morbid scenery. The later ones makes the world different but it's definitely interesting on its own.
The problem is really the writing. Planescape Torment tells a very good story with very good writing. Numenera wants to do the same. But instead they should have focused more on the rpgs aspects instead of thinking of Torment only as interactive fiction. You can design good gameplay but you can't enforce good quality writing that captures the players mind. the later one might happen or not despite having good writers on board. And imho good rpg mechanics (exploration, chargen, leveling, loot and combat) can help a good portion of capturing players into the game and into the story/writing. Otherwise you're better off with books or movies really.