The Trial of a Timelord is the first time since Mawdryn Undead (I think, there are a few that I've not watched since then), and only the second time in the 1980s that the show has attempted anything other than straightforwardly linear storytelling (not even a character having a flashback!), as the Doctor's trial is interspersed with evidence from three recent adventures. The entire fourteen-episode series makes one long story, with three other stories embedded into it.
The trial scenes are not very good. They try to make a big deal of how the Doctor's life is in jeopardy, but the staging and the Doctor's petulant attitude to proceedings (contrasted with the inner story where he seems to have graduated to a more subtle form of wit) makes it difficult to take this seriously as a threat, and it's easy to take sympathy with the Valeyard rather than the Doctor. Another problem is that it can't seem to decide whether the Doctor is on trial for interfering or interfering harmfully: if it's the former then they don't need to go into so much detail, if it's the latter, than the evidence is rather poor.
The trial format suffers from the use of the Matrix. We are used to how this ought to work: witnesses come on the stand, they start telling their tale, and then the screen wobbles, you are there with a voiceover, and then things start happening, the VO goes away for a bit, until they want to get out of the flashback... Course, with the Matrix (which is effectively the same as them watching episodes of Doctor Who), they can't do any of that, and the interruptions from this can be rather jarring, especially as they usually just cut straight to the reaction, rather than having a shot or two of the Doctor watching the telly and reacting to something that we've seen him see.
As for the inner story itself: I liked this. It managed to be funny, tense, and intriguing in turns. Glitz and Dibber were well-written and well-acted, a highlight. The Doctor and Peri's relationship appears to be being cast where it ought to have been in the first-place: they have friendly banter rather than the Doctor just being mean to Peri. Queen Katryca is good too, especially in her first scene with Glitz, where she shows that she has the one-up on him. The inner narrative itself is nicely structured, with the backstory being eked out gradually rather than being infodumped. At first we don't know Glitz's mission, or why Drathro is there, but this is revealed gradually and then eventually come to coincide. Imagine how dull it would be if we'd known all that (and Merdeen's backstory) up-front. Nice pacing and editing, too.
The only real fault I can find with it is that there's no need to invoke a threat to the universe (something that Terminus also just randomly threw in there for no logical reason - it'd be interesting to compile a list of stories where this happens - I mean, you can object about this being overused in the modern era, but at least they make a big huge deal about it, rather than just casually tossing it in as a line or two). Oh and as a nit-pick, didn't we have a secret ruler of a society who communicates only through CCTV only two stories back?
Of course, that final infodump doesn't come - the Valeyard has censored it, for reasons of national security! This would appear to be part of the overall plot, then. We'll see! This would probably be a stronger story stripped of the Trial scenes, but then that bit would remain unresolved... (also I think an attempt to edit one from this material would probably end up with jump cuts).
Some little touches
*the incongruously good model shot at the start of part 1 - they must have spent some absurd amount of money on that. Nice, but possibly better spent elsewhere
*the Doctor seems to be about to casually reveal his name in part 1, before he is interrupted by Peri!
*"all these subways seem the same to me", which we also heard variants of in 'Timelash' (corridors), 'The Mark of the Rani' (mines), 'Vengeance on Varos' (corridors) and Attack of the Cybermen (tunnels). Seems to become a habit
*they've gone to OB tape finally, getting rid of the jarring film look/video look change for different segments. This is one story though that could have done with different looks for different bits: the grading between the Trial and the Underground part could have done with being more different (both studio, mind); or possibly Colin's coat could be different in the Trial scenes