Richard Morrison was pretty ignorant of the design philosophy and its quite decent potential, then!
As others have said, it is simply a cluttered and badly-managed (in terms of layout) mess now, not running in accordance with the streamlined nature of modern travel for which it was designed. Property management now seem to think of it as a blank canvas and as a cash cow, and therefore the public think of it as the equivalent of a concrete cow - useless, ugly and a bit of a farce. I can understand London station retail rents being attractive things to gather, in one form or another, but I can't fathom why it's been done that badly!
There was recently a feature on Euston at the Destination Stations exhibition, which was adjacent to the Shop at the NRM York. The architectural models and the drawings showing the way the station was designed to fit in with the contemporary buildings around, as well as the public spaces, were a good lesson in how to do brutalism (well, things inspired by it) correctly: enough to make you think, not enough to make you hate.
The main building at Euston is bold, but it is hidden because it was deemed acceptable to do so; alas, this induces a feeling of neglect, far worse than controversy as a visible landmark. I know a number of buildings designed with similar philosophies; the more you can let the public see them, let the light in and allow them to be public spaces, people become curious rather than resentful. A clean, tidy, decluttered, open Euston with space for its people - its passengers - would be a beautiful thing today. It is a precisely-designed example of a building which should function well, and should look characterful but cleanly-proportioned, but can't.