There are two forms of pressure - internal and external.
I was moreso referring to external pressure, based largely on expectations put on them by the media and the general public.
Example: JaMarcus Russell is under a ton of pressure, #1 pick and he could be out of the league after this year if he plays like he has. But, he sucks ass and nobody expects much from him or the Raiders anyway so if he fails nobody will be surprised.
On the other hand, Philadelphia wants to crucify Donovan McNabb after every bad game he has. He is getting up there in age, probably has 2-3 more years in him at best, and has never won the Super Bowl. Not to mention Westbrook is now officially gone and he will be tough to replace and there is a lot of talk comparing the McNabb/Kolb situation to the Favre/Rodgers situation.
There's also a ton of pressure on Favre because of all the shit he has pulled and the fact that he just played his best season of his entire career. If he fails, he will get more attention than anyone else. He is a star, the spotlight is always on him, so everything he does is magnified by 10.
When people expect a lot out of you there is a lot more pressure on you. Everybody expects Brady, Manning, Romo, McNabb, etc. to make it to the playoffs and Super Bowl.
Nobody expects Jay Cutler and the Bears to make it. The underdogs don't have as much pressure on them as the favorites.