How is that cliche, it is a fact. Is it a huge coindidence that all the great dynasty teams have superior offense lines. When Terell Davis career was done they put an unkown Clinton Portis behind there and he managed to be just as dominate as Terell Davis playing under that same core offensive line. Since Portis got traded to the Redskins his productivity has dropped. He plays under an average o-line and right now he is just an average back in the league. That wasn't the case in Denver.
Sorry, but that's a terrible comparison. Clinton Portis is a stud running back in the NFL, and has been for his entire career. From a pure statistics perspective, Portis' best year came in 2003 with the Broncos, but he was also playing for a Broncos team that went 10-6 and had a solid Jake Plummer at QB, Shannon Sharpe catching balls, the great O-Line, and an above average Defense. In his career with Washington he's never played in a very stable system (many thanks to Daniel Snyder), with crap QBs, a mediocre O-Line and at best an average to above average defense.
Portis has been the ONLY option in DC and excluding the one season he was injured and missed half the games, he's averaged 1400 yards a season, 10 TDs, around 4 yards per carry, and 35 receptions for another 350 yards. He also plays in the best defensive division in football against 3 teams that pride themselves in stopping the run.
Also remember, Portis is now in his 8th season in the league. The average RB's career lasts about 3-4 years. He's certainly not among the top 5 backs in the league, but he's also not an average back.