He's pretty much the player Ovechkin haters like to pretend Ovechkin is. Largely one trick, subpar defensively (much more of a negative for a center than a winger), nonphysical, doesn't offer much playmaking creativity. Legitimately bad in the playoffs. Probably a 65-70 point player in a healthy year if Drouin can replicate what MSL was doing for Stammer. Tampa should have fetched a king's ransom at TDL for him and used assets/cap space to get more triplet/Killorn/Drouin type guys.
Stamkos is average defensively, not subpar. His biggest mistakes are on the offensive side when he thinks too much.
He's not overly physical, but he's one of the strongest players in the league and puts in the work on ice when he has to. Not every player is physical or has to be.
He's not 'legitimately bad' in the playoffs. He's average on a team with big time playoff performers. That's not that bad. Maybe without them he would have more fire to step up. Look at Hossa as an example of this.
His problem isn't that he's declining, it's not that St Louis made his points for him (if anything I'd say it was 50/50, except the art Ross year, Marty was on fire.) Being played right wing doesn't help, but that problem seems mostly corrected.
His problem is that he wants to be a playmaker, a 200 ft player and an overall better player. Don't get me wrong, he has improved defensively and that's good, but he thinks way too much about trying to pass, trying to carry the puck in, or trying to play defense that he forgets how he was supposed to play; that supposed 'one trick pony'. Seriously, he hesitates on perfect opportunities looking for a pass, a second or two earlier with his shot usually makes a goal. He also doesn't drift into the empty spaces as much as he did before. He's not even getting covered harder, he just doesn't seem to look for it.
It's 100% mental and he needs to look to rush and shoot, drift and shoot, and just give others the puck. He's literally the perfect player to have as a one dimensional monster.