Throughout the original Star Wars Trilogy we have got a sense of just how powerful Jedi can be. For one we only see three, two with lightsabers. None of them actually die in a fair fight, Obi-Wan willingly sacrificed his life to gain Oneness with the Force. We see Luke go through countless battles, on Hoth in ESB, Tatooine in ROTJ and even some on Endor. Luke is almost killed by Vader but is saved by the power of the Force and by Han Solo and ends up destroying the whole space station. Luke comes close to death with a walker but ends up destroying it instead. On Tatooine Luke takes down a many of the greatest villians of the galaxy and destroys the sailbarge with the now strangled to death Jabba on board. On Endor we see one of the only uses of a lightsaber to deflect laser bolts in the OT when Luke destroys a speeder bike.
The point being, in the OT the Jedi are painted to be invincible. The Force is there supreme ally and it will guide them over any obstacle and it will give them strength even when they are dying. But...

In the Prequel Trilogy we are indroduced to a new idea about the Jedi. In the Old Republic things are different. There are thousands of Jedi, spread across the galaxy working as the peacekeepers of the Republic.
As TPM begins we see two Jedi in particular- Qui-Gon Jinn and Obi-Wan Kenobi. We see them even avoid being gased to death in the Trade Federation conference room. They avoid death in the planet core by trusting in the Force to guide them. They even escape all the way to Tatooine on the pure essence of the power of the Force and a little luck. The Jedi are still invincible.
But on Tatooine we see a new side of the Jedi-

"I saw your lasersword. Only Jedi carry that kind of weapon."

"Perhaps I killed a Jedi and took it from him?"

"I don't think so. No one can kill a Jedi."

"I wish that were so."

This line in the movie serves to foreshadow Qui-Gons death in the end but it has a larger meaning. This sequence early in TPM sets the tone for the rest of the prequel Trilogy. Jedi can die, even normal men and soldiers can kill Jedi. This line sets up perfectly the fall of the Jedi order in Order 66.
It is so interesting that Anakin is the one who says this line. How quickly Anakin will learn how to kill Jedi. In only 13 years this young and innocent boy will raid the Jedi temple and destroy the Jedi Order.
And as Qui-Gon responds to him saying, "I wish that were so." it is almost as if he can see the events the Jedi have coming for them.

"A thought...anyone?"