He was an awesome talent, for sure. To this day, Leetch is one of the greatest "in tight" skaters I've seen. He seemed to be able to skate in a phone-booth, as it were, in full control of the puck, able to shift his body left, right, forward, backward, all with ease. I've rarely seen players so 'nimble'.
I think there's also a tendency of today's fans to judge players of the past by today's standards, which is usually a mistake. For the same reason people think Coffey was poor defensively, Leetch gets painted that way. But it has to be understood that when Leetch entered the League in 1988 and immediately became his club's go-to PP and offensive defenceman, he was thereafter expected to lead the attack and carry a lot of the load offensively. His coach(es) wanted him to rush the puck and take some gambles offensively. But this is sort-of incomprehensible to some younger fans.
Naturally, then, when the Rangers had more of a stacked line-up and stronger roster from around 1991 to 1997, Leetch looked best because he didn't have to carry that load so much (though he still did more than enough, winning a Conn Smythe!). I've seen Ray Bourque accused of being defensively weak(er) as well, by fans who don't understand that the game was different then. Go back and look at Boston's Conference Champion 1988 squad (which didn't even have Craig Janney for most of the season) -- if Bourque didn't lead the attack, who would? Oh, and he also had to be the top-defensive guy and shut down all the other teams' best players at the same time that he was expected to lead the team in scoring.
Some of Leetch's coaches, after 1997 or so, are probably guilty of encouraging his offensive game more than they should have, or maybe Leetch had some difficulty adapting to a weaker roster and the DPE. But he was very capable. I recall one shift in the 1996 World Cup where Leetch went into the boards against Lindros, hit him cleanly and hard, and easily stripped him of the puck. It was good, tough, 'Defense 101'. He could do that.