That question truly makes me feel old! And it assumes right from the start that he was a poor defensive player - which is so un-true as to be laughable. Klima! Ha! Klima was tossed off that team because he wasn't the "right stuff." If you ever get a chance to see the documentary "The boys on the Bus" (1987) you will see a side of Wayne Gretzky that is often unknown to the casual (or I'm assuming younger) fan that never saw him play. Gretzky was team captain of arguably the best team in a generation. If you will indulge an old timer ...
Gretzky was often maligned as a poor defensive player early in his career. Imo, partly out of spite - because he was ridiculously great - no hyperbole here. (And, there are and have been many "great players" since and loads of hype around them, but Gretzky was great like "The Beetles are the best band in the world" great - in short total hysteria) and partly because he was radically changing the way the game was understood and played. Cherry hated him as I recall, because "he wore a visor!" Uhhh, I like Don and all, but even Don recognized later that he was waaaay off base early on. Don has a hardon for Bobby Orr and that's likely where his bias comes from.
As to Gretzky's defense specifically:
They said he couldn't skate very well, until he would catch fast players such as Steve Bozek, Steve Tambellini or Theoren Fleury back-checking on a break away. Some said he "wasn't good at face-offs" and then he dominated Doug Gilmore, Joe Neiuwendyke and Bryan Trottier in respective playoff series. (for those that are younger, those were the best of the 80's and early 90's)
Gretzky's amazing anticipation and knowledge of the game meant that he ALWAYS (seemed) to KNOW where the puck was going. He anticipated better than any player since. His defense often looked more like a football defensive back waiting on an errant throw. He would pounce on a poor pass, force a mistake because he played so well positionally, was such as threat to score and often recovered loose pucks with no one else around.
Because he was on the ice for 20 - 25 mins a game in offensive or even strength situations, he rarely took shifts killing penalties, unless the oilers were taking a ton and he needed minutes to keep loose. Then he proved to be equally good (a super star) at PK with his short handed play. I remember one season he and Kurri played the forward positions on the pk and were leathal with SH goals (I had to look it up - '83 he had 12 SH goals, '84 he had 11!) Kurri had similar numbers.
One "classic" Gretzky defensive tactic was this: when an opposing team (such as Terry Crisp's Calgary Flames) put a "shadowing player" on Gretzky (it had to be a smart player with great agility and wheels) Gretzky would merely skate over beside another Calgary player and take two players out of the action. You would see Gretzky and his shadow, shadowing another valuable Calgary forward in a little useless knot on the ice. It was frustrating as hell for the other coach, the other players and it frequently left his line mates often with a man advantage.
Sure he wasn't a banger, but he was defensively excellent on the boards (like the Sedins are now) but even better because he was just that much better a player. He used superior body position, a lightning quick stick, and a shifty first move to seperate the puck from the offensive player, and himself from the pack. He rarely got touched, and if a forechecker pursued too hard or over-committed, his ridiculously good vision on ice and passing skills sprung an open player for a scoring opportunity. Fore checkers soon covered the guys he might pass to and tried a kind of zone D on the Oilers.
He was frequently vilified for being a "seagull" but imo he just had waaaay better anticipation of the play, and was already turning up ice AND available for the break-away pass. Playing up-ice meant that a player on the opposing team had to cover him and frequently it pulled a defenseman AND a forward out of the offensive zone. Imagine playing 3 on 4 offensive most of a game - not particularly effective as it turns out.
I remember guys screaming at the tv set yelling, "why doesn't someone hit him" and claims that "there's a league-wide conspriracy not to hit Gretzky." etc. None of it was true, he was JUST THAT GOOD. You couldn't get a piece of him. You couldn't tie him up, knock him down (legally), or stop him. He played double shifts tirelessly and was on ice 25 mins. a game in all circumstances.
It is easy now as time passes to forget and discount his abilities. The guy we saw and remember now from the St Louis and New York days is a shadow of the player Gary Suter jammed into the boards. Sure he was playing against Andy Schliebner (look him up) in those days, but he was UB-BUH-LIEVABLE in all facets of the game.
I only regret, that as a long-suffering Canucks fan, I spent so much time hating his guts in the early days I never fully appreciated how truly great a player he was.
finn