First and foremost is Clarke's defensive domination. Clarke played big minutes, many times matched up against the other team's biggest threat, in a league where scoring was around 10% higher than today's NHL. In this environment Clarke was on the ice for 14 goals against at even strength, an insanely low number. Clarke had elite linemates in Barber, a low end HHOFer, and Leach, a non-HHOFer who had talent to peak at a HHOF level, and had an elite goaltender behind him in Parent, but he did not have an elite defenceman on his team. To put this into perspective somewhat, Bergeron's low number of even strength goals against in a full season is 34, and he did that in a significantly lower scoring league (1975 NHL was just about 25% higher scoring than the early 2010s NHL was) with, like Clarke, a strong team around him and Chara. Datsyuk's low number of even strength goals against in a full season was 29, with a strong team around him plus Lidstrom and in a lower scoring league. Kopitar's low is 30 and Toews' is 34, and again each of them had an elite defenceman behind them. Going back further, Fedorov's low in a full season is 33 and Trottier's is 35. Even Carbonneau's low is 29, playing in a somewhat higher scoring league but playing less heavy minutes plus with Roy and Chelios behind him. This past season's Selke winner, Barkov, had 35 even strength goals against. Roughly 30 goals against at even strength seems to be the lower limit for elite defensive centres in the NHL over the course of a full season over the last several decades, except for Clarke. Finally to add to this, Clarke was an elite penalty killer, generally regarded as the best in the NHL at the time. NHL coaches were polled in 1974 and 1976 and both times Clarke finished first in the category of best penalty killer.
While I am comparing Clarke to recent Selke types, it is important to note that he is also a level above any of Bergeron/Toews/Kopitar/Barkov offensively. Clarke finished sixth in scoring (and 16 points ahead of seventh) behind Orr, Esposito, Dionne, Lafleur, and one point behind Lafleur's linemate Pete Mahovlich. Clarke was also able to combine his elite offence with his peak defensive impact, which is the main argument in his favour. Clarke was on the ice for 91 goals for and 14 goals against at even strength, so 6.5 times as many goals for as goals against. That is an incredible rate. Looking at the very best forwards in the NHL since 1960, not just Selke types, none of them come close to Clarke's result. Bobby Hull peaked just over 3, Esposito peaked just over 2, Lafleur peaked at 3.25, Trottier peaked at 2.8, Gretzky peaked at just under 2, Lemieux peaked at 2.33, Yzerman peaked at 1.75, Jagr peaked at 1.92, Sakic peaked at 2.23, Lindros peaked at 2.05, Forsberg peaked at 2.47, Crosby peaked at 2.17 (shortened season), and Ovechkin peaked at 2.34. Obviously some of this is era dependent, but even just looking at stars from the 1970s Clarke didn't have an elite defenceman behind him like Esposito, Lafleur, or Trottier did for their peaks.
I won't pretend that a rate like (even strength goals for)/(even strength goals against) is the be all and end all, both because games are not generally decided by rates and because other players have an impact, but Clarke fares very well in absolute terms as well, and his +77 goal difference in goals for and goals against at even strength compares very well with almost any other elite forward. Trottier's best was a +79, and that was with peak Potvin and Bossy. Lafleur peaked at 91 on probably the most dominant team ever (and hall of fame linemates and three hall of fame defencemen plus a hall of fame goaltender) but only surpassed Clarke's +77 the once. Esposito peaked at +72 playing with Orr. Gretzky peaked at +84 (and only surpassed Clarke's +77 one other time) and Lemieux peaked at +68. Jagr peaked at +41, but playing in a lower scoring era limits opportunities to stand out in this way.
Overall Clarke's season was wildly impressive at even strength, and he also happened to be possibly the best penalty killer in the NHL. The only real critiques I can see for Clarke are that he had 125 penalty minutes and that he wasn't at the top of the league on the power play. That was a high penalty minute era but Clarke still finished 23rd in penalty minutes and well ahead of the other top scorers, a clear negative. Clarke was only 14th in power play scoring, but this isn't quite the issue that it might appear to be. Part of the reason Clarke was only 14th in power play scoring is that several players from the same power play units finished ahead of him - 3 from Boston, 3 from Montreal, 3 from Buffalo, 2 from New York - they weren't all better power play scorers than Clarke, whose nearest teammate was 32nd in power play scoring. Philadelphia was also the team with the fewest power play opportunities in the NHL - 257, which was 16 fewer than the team with the second fewest opportunities and 115 fewer than the league leader. If Clarke played on a team with an average number of power plays and with a top offensive defenceman he's probably knocking on the door of top five in power play scoring.
