Wow ... to me you're giving Duff way too much credit.
This is a guy who had a couple OK (50-point) offensive seasons early in his career on terrible Leaf teams, and then averaged about 35 points/season from age 23 onward, and was a third-line type player for the majority of his career. Yeah, the eras are different, but Nieuwendyk was clearly a front-line star-calibre offensive player in this league for many years.
Duff is almost a carbon copy of Bob Bourne, career-wise. Quality two-way forwards who won a boatload of championships as support players and had a knack for raising their games in the playoffs. Almost identical career numbers when you adjust for era. Bourne's resume is to me actually a bit more impressive, and he isn't even close to a HHOF guy.
And Bob Nystrom is another guy off that same Islander team with a very similar career resume to Duff. It still blows my mind how this guy was inducted ... you can't find a legitimately comparable guy who has even received a sniff at the HHOF.
Hence my argument for Esa Tikkanen. Though I think you may be under-rating Duff. Esa was a great Defensive forward and was the ultimate pest and was a top playoff performer. Was integral to multiple Cup teams and probably the best player on 2 weaker Edmonton teams that made it to the 3rd round. Plus he was on the first line of Edmonton with Gretz and Kurri not on a 3rd line.
Of course I personally witnessed Tikkanen's greatness while Bourne and Nystrom were a bit before my time and Duff far before. I think as time goes on and the guys on the Hall of Fame committee were actually teammates/on ice opponents as coach or player of some of these more recent players they may get in like Duff did. It is generational. I witnessed Tikkanen and how amazing he was in the playoffs for Edmonton every season. Many did not witness Duff and all the things he did to help the Habs and Leafs win games. The intangibles you can't see on a stat sheet and really can't comprehend without actually watching the players play games is the reason, I think, that Gilles, Duff and even Neely are in the HHOF.
A player like Nieuwendyk might not get voted in anytime soon. He might, but it really depends who is on the Hall of Fame committee each year. He could be like Duff and have to wait a very long time to get in.
But Nieuwendyk, Tikkanen and many others are guys that maybe will get in a long time from now when Messier or Gretzky or Hull are on the Hall of Fame selection committee. I think that having several key Islander people on the HHOF selection committee is the reason why Gilles got in.
As time goes on I care less and less about perhaps less prestigous players getting in the HHOF. Federko, Gilles, Neely were great players. If they get in the Hall then that is fine, even if quite a few question their selections. Anyone that watched the Islanders win their 4 Cups is not going to argue much with Gilles selection I think.
The biggest people that argue against Duff and Gilles are people looking back at the stats and saying their stats aren't good enough. But stats are so over-rated in evaluating hockey greatness. Some day people will look back and sat "Why the heck is this Bob Gainey guy in the HHOF? he got 40 points when the best players were getting 100 a season?". Of course those that saw Gainey play won't argue his merits but the stat people will.
I like stats a lot but Hockey isn't baseball and they tell far less of the story in Hockey then they do in baseball.