The thing that jumped out at me, and the big difference between Clemens and Bonds- Bonds did not file a defamation lawsuit. The suit opens Clemens up to cross examination and allows the defense to pry into his personal life. If he has something to hide, it'll come out in court for the defense.
Defamation lawsuits are no joke. For a statement to be defamatory it needs to be specific, untrue, published to a third party, and most importantly- the prosecution needs to prove damages.
I studied a lot of 1st amendment law last year, and the fact that Clemens has filed the lawsuit carries a lot of weight. Most celebrities never do it in something scandalous. Kobe was found not guilty of rape, which was a pretty damning charge the girl put forward. It cost him hundreds of millions of dollars in endorsements and killed his reputation. Kobe never filed a defamation suit because then the burden was on him to prove the girl was lying.
Same situation now. If Clemens used steroids, he's to have a hell of time proving he didn't to a jury. The burden is going to be on Clemens to prove McNamee was lying, and it'll be interesting to see how far this progresses.
To quote my professor- the ultimate, unbeatable defense to slander and libel is the truth. A case like this, the one that proves to be telling the truth wins.