It was, coincidentally, during the making of Full Circle that Baker decided not to return for an eighth season -- but that's not a commentary on the serial itself, which (at least on the basis of these two episodes) is one that's generally firing on all cylinders. There's a cleverness and momentum about the script that makes it very entertaining, and there's also a sense of drama that was missing from the last two stories. Romana's unhappiness at being summoned back to Gallifrey is understated but affecting, and the main scenario at play on Alzarius -- the coming of Mistfall -- is nicely compelling too. Mistfall, much like Alzarius in general, is presented in an interesting way, and writer Andrew Smith makes a good move by having a band of rebellious Alzarians, the Outlers, remaining outside the Starliner, as this means that there are people for Romana to interact with outside the ship while the Doctor is inside. The main outsider Alzarian, Adric, is passable -- some moments are a little wooden, but Matthew Waterhouse also has a few rather good moments: his waking up in the TARDIS, alarmed about Mistfall, is one such moment.
Mistfall is treated well, as I mentioned before, and not only is it presented as a serious concern for the locals, but it's filmed in an effective manner as well -- all the bubbling mist that hangs over the place at the end of part one and all throughout part two looks fabulous. Plus, it really helps sell that first cliffhanger, as the Marshmen rise from the water. The Marshmen also look great, and there's some fine acting going on from the people inside; the Marshchild that follows the Doctor inside the Starliner is especially good, and the fact that the Doctor can see that the Marshchild is terrified but harmless is another great moment.
Really, Tom Baker in general gets to be very good here, even in exposition scenes with K-9. And his first meeting with the Deciders is also great: "Why can't people be nice to one another, just for a change?" the Doctor says, after noting that the Marshmen dragged Decider Draith under the water. "I mean, I'm an alien, and you don't want to drag me into a swamp, do you? ...You do." Romana fares less well: she maintains her cool under pressure, after the Outlers attempt to hijack the TARDIS, but the cliffhanger to part two involves her trying to defend herself from some large spiders by grabbing a riverfruit (a type of melon), only for a spider to hatch out of it and attack her. Which might be excusable if she hadn't just witnessed a bunch of spiders hatching out of other riverfruits. And K-9 does well for a bit, but then he enters a cave and loudly announces his presence to the Marshmen, which leads to one knocking his head clean off.
But overall, it's an entertaining story and a compelling script. If the last two episodes are like this, then Full Circle will be something special indeed.