There's a certain phenomenon that I think all writers experience sooner or later. Whether in writers' forums, or through emails, or in person, someone eventually comes up to you and says, "I've got this great idea that I think would make a neat book. Now I'm no writer, so I know I can't do it justice. But your writing is really good. Would you be willing to write it for me?"
This demonstrates both an amazing amount and an amazing lack of self-awareness. On the one hand, they understand that ideas aren't the important part of being a writer. The expression of them is. Hamlet, boiled down, is "stepdad killed original dad, son kills stepdad in revenge." That same plot can be one of the greatest works of literature in the English language...or it can be a Lifetime original movie starring Eric Roberts and Kirk Cameron. The concept is only the beginning; turning it into an actual plot requires subtlety and deftness of touch, and working the plot into a functional story requires an ear for dialogue, a gift for phrasing, and an understanding of human nature. Not everyone can do that, and it takes a lot of humility to get that. (By contrast, it takes a remarkable amount of obliviousness to miss the fact that every good writer has about a dozen ideas on a notepad somewhere for every story they write, and they really don't need more.)
Why do I bring this up? Because 'The Pit' was clearly written by one of those people who wasn't a writer, but had lots of ideas. Practically every single one of them made it into the book: A planet contaminated by a mysterious "red weed" that slowly spreads, stopping time for everything inside it; android hunter-killers chasing shapeshifters who've stolen a bomb that can obliterate whole solar systems; a time-displaced William Blake helping the Doctor hunt Jack the Ripper; a forbidden planet that's secretly artificial; telepathic police and criminals who defy them by refusing to think about their crimes; an ancient Gallifreyan who's the last survivor of a war that the Time Lords wrote out of the history books because it was caused by the arrogance and stupidity of Rassilon; a cult that has spread through time and space that worships the Time Lords' ancient enemies in that war; a drug extracted from another dimension that creates perfect, inhabitable hallucinations; oh, and the whole thing is also trying to work as both an explanation and a critique of Manichean philosophy and the eternal question of free will.
But in practice, the whole thing turns into a muddled mess. Every situation devolves into almost-total stasis; the Doctor and Blake spend much of the book wandering through time and space with no purpose or destination, and Benny is pretty much just a passenger as well. The book spends big chunks of time on a murder mystery and a civil war, neither of which goes anywhere and both of which are simply abandoned when the novel ends with the solar system blowing up. (Which would be spoilers if, y'know, we weren't told at the beginning of the novel that Benny and the Doctor were investigating a whole solar system that blew up.)
It's astonishing how early the novel establishes the key situations (androids hunting shapeshifters, civil war raging on planet Nicaea, Doctor and Blake lost in space-time, Benny a prisoner of one of the androids who was separated from the pack, scientist's wife on planet escaping the red weed, shapeshifters heading toward a ruined castle to make use of their doomsday bomb) and how late it is before any of these plots actually begin to move. Characters talk and talk and talk, they walk and walk and walk, but it's maybe page 200 before the plot actually moves. And even that wouldn't be such an unforgivable sin, if not for the actual talking we get:
"'Do you believe in the Prime Mover?' Brown asked. Kopyion didn't answer. 'I just wonder about the future. Will there be any beauty, or is there just aching, longing loneliness?'"
People have called Ayn Rand didactic, but she's Noel Coward next to 'The Pit'. Nobody comes off as a real person, only a vehicle for the author's speechifying. The Doctor and Benny come off worst, because we have examples by other authors to compare them to, but they're not actually written any worse than anyone else--we just notice it more because they're both behaving so badly out of character. Penswick was clearly trying to make a point about perils of treating the Doctor as an unimpeachable authority source by giving us an even older and wiser Gallifreyan who sees the Doctor as naive and foolish...but again, there's a lot of work that goes into the translation of that idea into an actual story, and Penswick doesn't do it. He's forced to make the Doctor less competent and intelligent than we've ever seen him, simply so that Kopyion's "I'm going to blow up an entire solar system to show the Yssgaroth how tough I am" plan sounds sensible by comparison.
People who defend 'The Pit', and there are some who do, point to its influence on the series. A lot of its ideas about ancient Gallifrey and the wars it fought against creatures that would be inimical to the very order of the universe have become accepted as a general underpinning of the history of the Time Lords. But those ideas became accepted because they were better written later, by writers who turned that basic notion into something interesting and meaningful. 'The Pit' is less a fully fleshed out novel in its own right than a collection of ideas that Neil Penswick presented to present and future Doctor Who authors...saying, in essence, "I've got this great idea that I think would make a neat book. Now I'm no
writer, so I know I can't do it justice. But your writing is really
good. Would you be willing to write it for me?"
What reputation 'The Pit' has rests on the fact that eventually, somebody did.