The biggest single fault was that there were only two escalators to conveniently serve the MML platforms. When the rebuild opened, there were massive queues when a full HST arrived and everyone tried to use the single down escalator. So both of these escalators now run Down, and passengers trying to go up to the MML platforms have to use an Up escalator halfway down the Barlow shed towards Euston Road. Which is an inconvenient and non-obvious diversion for passengers from Kent or Thameslink, or from the northern Underground hall. A bank of three escalators here could have run one up and two down to solve the problem. Or the Up escalators could have been located closer to the platforms they now serve.
I am far from convinced by the practicalities of this. One only has to go to somewhere like Waterloo (main concourse to/from the principal underground entrance,
not the Jubilee Line) or Holborn (foot of the four escalators between the Central and Piccadilly Lines) to see how having escalators running in both directions
in a locations where there are multiple flows and approaches at top and/or bottom can cause major congestion and significant risks. A more diffused and quasi 'one way' system is generally preferred. The recent enlargement of Victoria Underground station shows this well.
It is also necessary to mitigate against people stepping off an escalator and immediately stopping to consult departure indicators or fumbling for tickets to present at barriers.
(I may be biased as my commute involved continuing on foot to/from south of the Euston Road so there was always a range of suitable station entrances and it didn't really matter where in the Barlow Shed I ended up changing level in either direction.)
Something that seems to be missing in this thread is a general appreciation that with unprecedented passenger volumes, whether around King's Cross/St Pancras, Birmingham New Street or elsewhere, it is essential to just spread people out for reasons of both comfort and safety. A 'bee line' for everyone is simply not possible at a location handling up to a million journeys per week.
At King's Cross/St Pancras a thoughtful glance at a map can make it immediately obvious that the complex
must operate at at least six levels: one main line approaching over a canal; one main line approaching under the same canal and four underground lines that cross at the same point, which clearly cannot be at the same level as each other. (Thameslink is a bit out on its own but is arguably a seventh level.) So there are always going to have to be loads of stairs, escalators, lifts and ramps, not to mention entrances and exits and a couple of subways to cross busy roads.
In most cases, if you use a station once you are likely to use it again (if only for a single return leg). Whenever I go to somewhere 'new' (or that has evidently changed materially since I was last there) I find that it is a good investment of my time to devote a few minutes to familiarising myself with the surroundings. This approach has always served me well and I commend it to others.