Always shocks me when people dismiss the basketball players of the past. First, Russell was obviously insanely athletic, so the notion that he couldn't compete today for that reason is insane.
Second, people bring up the idea that he was too thin. Clearly, anyone who has watched KG the past 18 years should learn that you don't have to be thick to be a dominant force in today's NBA. If anything, most bigs still are thin (and always have been, see Kevin McHale). It's really been that aberrations like Shaq and Dwight Howard who have skewed people's perceptions.
Finally, I'd say this: Kareem Abdul-Jabbar came into the NBA the year after Russell retired. I think it's safe to say that a year later, it was the same caliber of competition. Kareem did dominate right away. However, he continued to dominate even until his late 30s, and he was dominating the a league that had the likes of Larry, Magic, and Jordan. So if the NBA really got so much better in the '80s, how could a dinosaur like Kareem who dominated the Russell's era, continue to dominate so late in his career in such an "athletic" league?
In my opinion, the answer is because the league didn't actually get any better and that the old time stars of the '60s and '70s absolutely could dominate today.