This is quite a run of the mill adventure but I actually think it gets overly harshly judged. There are lots of qualities in this 4 part story as well as some less impressive aspects.
Strengths include Peter Davison who is in excellent form as the Doctor throughout showing great qualities of decency, determination and bravery. Adric and Nyssa are in their best form too I think and both play their roles very well. Tegan show a bit of grit in taking action. Of course her actions are unhelpful and she is a bit of an annoyance on occasion but she is not too bad. The guest cast in my opinion is excellent. Stratford Johns provides gravitas and smoothness as villain Monarch and is a major plus. His delivery of lines is superb adding a real impressive quality. His colleagues are pretty good and Bigon played by Philip Locke is excellent creating a wise, endearing and deep thinking ally for the Doctor.
The plot is perfectly serviceable if unexceptional but I find the script really good in terms of dialogue. There is intelligence and thoughtful exploration of moral decisions presented. The effects are OK, the make-up is decent for the day but the real plus in terms of the production is the fantastic sets. The look of the sets is great and there has been great effort in creating all the wide ranging areas with different machines and so on. It really adds a sheen to the serial.
Now to the negatives:
The first episode is pretty lacking in real content and has a rather silly scene when Tegan does a 'sketch' to show Earth fashions of her day. The result of her sketching is a ridiculously detailed bit of draughtsmanship (even commented on by Monarch). The second episode continues to be rather padded with the ritualistic 'entertainment' and slow development of the plot. The final episode contains an attempt by the Doctor to jump from the ship to his TARDIS across a small distance through space. This is an impossible scene to achieve believably with the budget and other restrictions of the time and whilst it is a pretty good effort and is rather fun there are obvious shortcomings. Across all 4 episodes there is also the slightly odd willingness of Monarch to allow his 'guests' to travel freely around plotting against him. It is made just plausible enough for me by Monarch's clear arrogant sense of invincibility and him falling for a few of the Doctor's tricks but it still seems slightly strange at times how much freedom the Doctor gets to foil Monarch's plans.
The third episode I think is really really good. It has hardly any issues, has most of the story development and has very good dialogue and acting. The other 3 parts are OK but not all that great.
My ratings: Episodes 1, 2 & 4 - 7/10, Episode 3 - 9/10, Overall - 8/10.