I see Coffey as being underrated. he's easily the second best offensive defenseman ever and I've always felt that his defensive shortcomings have been vastly overstated. To me great defense is getting the puck out of your end. Coffey did this as well as anyone.
You're probably right.
Coffey did the following:
-- 1st or 2nd-team All Star 7 times in first 9 seasons.
-- more goals in a season than Bobby Orr
-- more 'primary points' (goals or first assist) in a season than any D-man, ever
-- four season in a row at +50 or better
I personally think Coffey's greatest seasons were his second through his fifth: 1981-82, 1982-83, 1983-84, and 1984-85. His whole career sort-of peaked in the 1985 playoffs (esp. vs Winnipeg and Philadelphia) when he almost won the Conn Smythe. If we freeze his career in the summer of 1985, it looked like the was heading for an Eddie Shore / Bobby Orr-ish legacy. On the day he turned 24, he already had four 1st/2nd team All Stars, one Norris (should have had two by then), two 100+ point seasons (nearly three), two Stanley Cups, and he was +201.
He came back in 1985-86 and had his biggest offensive season and another Norris, but I personally think he had already peaked prior. In 1985-86, he would be on the ice for 44 of the 77 PP goals Edmonton allowed against (i.e., far too many), and a crazy 162 goals against, overall (a number he would surpass, barely, in 1989-90 in Pittsburgh). That was also the first season he started complaining about stuff (didn't want to play the Soviets, or the All Star game), and when he and Sather started butting heads. He really stopped playing defense competently late in 1985-86 because he was trying to break Orr's goals record (which he did, scoring 31 goals in the last 40 games!), but this came back to haunt him in the playoffs, when he was not effective.
He did 'tighten up' a bit in the '87 playoffs, though he was injured for some of it, but by then I think he was already pretty much done with Sather and Edmonton.