The Flick: Russell Crowe and Ridley Scott present the epic tale of Maximus, a Roman general who became a slave, a slave who became a gladiator, a gladiator who defied an emperor and an actor/director team who threw all the audience goodwill they'd earned on this away by releasing A Good Year six years later.
The Inaccuracies: Commodus, the hare-lipped Roman Emperor who lusted after his sister in the film, was in real life held in high esteem by the senate and ruled for a successful 13 years (rather than the ineffectual few months depicted in the film). Also, though the Emperor did, in fact, have an enthusiasm for gladiatorial combat (he did so incognito), he didn't get his ticket punched in the arena. He was killed in the bath by a wrestler named Narcissus to prevent him taking office as consul.
Why It Would Have Sucked Otherwise: No one wants to watch Russell Crowe take 13 years to murder an emperor, who is basically a decent guy, only to get beaten to the kill by a wrestler. We like our villains like we like our Books of Genesis: with implied incestuous relationships. Also, since any Roman unit that broke ranks when in combat against barbarians would have been mercilessly slaughtered, the movie would have ended within about 15 minutes.
The Flick: Mel Gibson's earliest example of "loose" historical reenactment, Braveheart marks a promising start to a career later spent boiling complex political issues down to "Mel Gibson kills Englishmen with an axe" (The Patriot) and curiously drawn-out torture scenes involving his heroes (The Passion of the Christ).
The Inaccuracies: Far from a scrappy commoner who clawed his way up from the mud to defend his homeland, William Wallace was actually a knight from a noble family, and his father Malcolm wasn't killed by the English, but fought on the English side in exchange for political favor. Also, instead of kilts, the Wallace and his army wore saffron shirts.
Why It Would Have Sucked Otherwise: We have to imagine that if Mel Gibson were forced to play a role any more layered than that of the just and righteous warrior-king-redeemer, his face would melt off from the challenge, revealing the circuitry within. And as entertaining as that would be, it's not as entertaining as the actual movie, or the years of mileage we've gotten out of screaming "They may take our things-but they'll never take our FREEEEEEDOM!" when we have our nail clippers taken away from us at airport security.