The main difference between Harden and Cousins is that Harden is in a position that helps him carry the team more. He is doing for the Rockets what Dwyane Wade did in his prime for Miami. Back then Wade used to be a prolific scorer but he also weighed in with around 7 assists per game, which was better than some starting point guards. Harden is averaging 28.6 points and 7.2 assists per game, the latter ranks him seventh in the league in assists per game, which is better than only five starting PGs have more assists per game. So not only does Harden carry the scoring burden for the Rockets, he also carries the ball handling and play making burden for the Rockets as well, and that helps him improve his team mates because their offense runs through Harden. Cousins might be a prolific scorer for us, but ball handling and play making is handled by other players and that can be very up and down. So perhaps that positional difference helps Harden elevate his team mates, whereas Cousins can only do so much in the role he has in the offense.
Another difference is that Harden tends to keep his composure on court. He's never been ejected and I don't think he's fouled out this season (?), whereas Cousins has lost his composure a number of times this season (eg. shouting at Karl during a time out, pushing a security guard), he's been ejected and fouled out, and players still manage to rile him (eg. when he nearly punched Steven Adams). Cousins is a fine player, but opponents know how to get to him and that sometimes affects his performances on the court because he loses his focus and loses his cool over things that shouldn't bother you. I don't know how much this affects his team mates, but it doesn't help his team mates when he loses his cool or focus. I haven't really seen Harden lose his temper, lose his cool, or lose his focus as much as Cousins. Again, Cousins is a fine player, but he's very temperamental.
For me, Cousins is a guy you can build a team around, but he needs a good supporting cast to be successful. We have some decent pieces like Rondo and Gay, but neither are really top level guys. Gay has always been a chucker that is capable of big games, but he has his fair share of bad ones and his defense is average. Rondo loves to rack up assists, but the other parts of his game don't match up. I think that's what has hurt us this season that we have a lot of players playing like individuals, rather than playing as a collective. That's why I don't think Cousins is the type of guy to carry a team on his back to the play offs because so far he hasn't done it. Maybe one day he does carry this team on his back like Harden is doing with the Rockets, or like Wade had to do at times in Miami, or LeBron in Cleveland, or Dirk for a lot of his career in Dallas. But right now I think Cousins needs a running mate like Shaq had with Penny Hardaway, Kobe Bryant, and Dwyane Wade to truly make use of his potential because those guys helped ran the offense and that helped Shaq do his thing. Those kind of players don't grow on trees, but if we could find more team orientated players rather than the individuals we have right now, I think we could get to the play offs and see Cousins get a huge boost from that.