It's difficult to comment on the technical changes without being a drummer, but from my perspective Pick moved from a jazz feel through to being confined to louder, straighter rock drumming, which he didn't like. To refresh my memory I turned to the Michael Oldfield book 'Dire Straits', published by Sidgewick and Jack on (if you don't have it then it is worth seeking out).
Here are two extracts on Pick which bookend the story, the first from when he joins the band, and the second about his departure:-
"In terms of experience, Pick Withers was worlds apart from the
other three, An early memory for him is seeing the Boys' Brigade
band marching down the street to the beat of a big bass drum. 'For
some reason I wanted to get that drum, 1 had to go through a lot ofred
tape - be in the Boys' Brigade for a year, then play the bugle for a
year. My great piece was playing "The Last Post" at camp. ThenI
had to wait for a vacancy and I finally got this drum. I used to drive
my parents mad.
At seventeen, he became a professional musician and joined a
band called the Primitives. For three years he kept the beat on tour
in Italy, playing a collection of R'n'B oldies and weak band originals.
On returning to England, there was a dizzying sequence of sessions
and a band called Spring, who recorded an album for RCA without
much commercial or critical success. The critics thought it was
pretentious, and in retrospect, it was.
He then moved to Dave Edmunds' Rockfield Studios in South
Wales, to a not particularly lucrative job as house drummer: 'You'd
have to ask for forty pence for fags.' However, it did give Pick an
opportunity to work with an impressively varied collection of artists,
among them Bert Jansch, Howard Werth, Michael Chapman and
Del Shannon
He met up with Mark at the house of a mutual friend, where, Mark
claims, 'He was busy starving to death.' When Pick came down to
Farrar House, the effect was immediately obvious to all. John says:
For me, as a bass player, to be playing with Pick Withers was
fantastic for a start. I'd never played with anybody as good as him.
I really felt very shy, initially, about playing. He used to play ina
folk club and he was very delicate - nowhere near how he ended
up, thrashing the kit, as he put it. A great player, then, and I think he
became a better player over the years, though he'd probably
dispute that. He certainly didn't like some of the aspects of
rock' n'roll, the fact that it had to be four to the bar. He didn't like to do
straightforward drumming too much".
And later...
"Ed says: 'We don't really know why Pick left. I think it's a
combination of reasons. It had been on the cards for a long time that
he was unhappy with touring; not the playing, but the time spent
away from home. During the making of Love Over Gold, Pick
conceived in his mind certain things that I don't think have much
foundation in fact- for instance, that drums no longer playedan
important part in Dire Straits. Another comment he made to me was
that: "If I can't play drums with Weather Report, Im going to give
up.'" Pick completed his work on the album and left the band on 23
June.
Mark says: 'He's in Wales with his baby and his wife and he wants
to learn vibes and piano. He said he wasn't happy with his drums.
Touring is tough. If we'd been eighteen when the first album started
going nuts all over the world, we'd probably be lucky to be alive
right now. I think it's only the fact that we had some semblance of
sanity early on that we managed to get through it more or less intact.
John agrees: 'One of the important things to remember is that
when the band became successful, neither Pick nor David could
handle it. Some people can, They take it for what it is, enjoy it and all
the rest of it. But David was never happy with it, although he would
love to be successful himself as a songwriter. Pick was the same. It's
something that comes from your upbringing - you prefer to be the
underdog, almost, rather than be on top."
Not sure I like John's angle really on this, maybe they didn't get on. Anyway Pick is a fabulous drummer.