Most of the GM observations on Fischer's Middlegame -- from 'Russians Versus Fischer' by Plisetsky and Voronkov -- are readily understood, except perhaps the first: 'He is highly skilled at playing with his pieces.' It's the sort of observation you would expect to find on any decent player. What was so special about Fischer's play?
The phrase 'skilled at playing with pieces' brings to mind 'I Play Against Pieces' by GM Svetozar Gligoric (Batsford, 2002). Here's how Gligoric explained the title of his book:-
Foreword: An invitation to write for the respected Russian series 'Famous chess players of the world', which included books on world champions and other top grandmasters in history, was an honour one could not refuse. And in 1981, my book with 105 selected games was published in Moscow with a printing of 100,000 copies.
It was called 'I Play Against Pieces' -- words taken from an interview I gave to the editor. The unusual title referred to chess as an art and a clean struggle of ideas, thereby trying to ignore the less dignified influence of psychology and personal conflicts.
That doesn't seem to be the sort of play that the Soviet GMs had in mind. Fischer was, after all, a master of psychology, as his last opponent in the Candidate matches acknowledged (see Petrosian on the 1970-72 Cycle, quoting from 'Petrosian's Legacy' by Tigran Petrosian):-
Now it was my turn to play Fischer. [...] Just look at the way in which he has been able to impose his will on the authorities, and get all his conditions. At the same time his opponents do not achieve the same results. It makes one uncomfortable to know beforehand that the town, the hall, the lighting, and even the furniture is designated by your opponent.
The meaning of the Soviet GMs becomes clear when their observation is given in full:-
[Fischer] is highly skilled at playing with his pieces. This accounts for his liking for positions of the open type and his clearly evident desire to avoid closed positions. Little wonder, therefore, that Fischer seldom locks the center in the Chigorin Variation of the Ruy Lopez,
A related point is made later in the same document.
Very often -- especially when playing with the Black pieces -- [Fischer] deliberately plunges into extremely complicated playing with his pieces, which can provide ample scope for his tactical resourcefulness, his 'calculating ability', and his remarkable skill at coordinating his forces.
Given this predeliction for open positions and piece play, Fischer's life long preference for 1.e4 makes even more sense.