When Jaques says "all the world's a stage," he draws our attention to the theatricality of day-to-day living and he also reduces human life to an acting role, which is a pretty cynical thing to do.
Of course, Shakespeare also draws our attention to the fact that "Jaques" really is nothing more than an actor performing a role on a stage. Shakespeare loves reminding his play-going audiences that they're at the theater – he does it in just about all of his plays, including Hamlet and The Taming of the Shrew. (Did we mention that Shakespeare's acting company built a theater in 1599 called The Globe…sounds kind of like The World, right?)
In fact, the title character of Macbeth says something nearly identical to Jaques' speech:
Life's but a walking shadow, a poor player,
That struts and frets his hour upon the stage,
And then is heard no more: it is a tale
Told by an idiot, full of sound and fury,
Signifying nothing (Macbeth 5.5.2)
Because Macbeth has just learned that his wife is dead, there's something profound about his hopelessness. On the other hand, when Jaques says pretty much the same thing, it just seems like a giant cliché being uttered by a poser and a wannabe philosopher. Even though what Jaques says seems to be true (the world often really does seem like a stage), it can be hard to take the speech seriously.