I was going to meta, but I think I'll let the screencaps talk for me.
Screencaps from 5x04 are from
oxoniensis and screencaps from 1x21 are from Supernatural.TV.
A couple things of note: The visual parallels between John giving Dean the Colt and Dean giving Sam the Knife are pretty apparent (though not shot-for-shot). And I think it serves a similar symbolic/thematic point. When John gives Dean the Colt he's also giving Sam to Dean, again. He's officially marking Dean as an adult and bequeathing his quest onto Dean's sole shoulders, in a sense (you finish what I've started, watch out for your brother). The only fundamental change in the relationship dynamic is that John no longer treats Dean like a grunt but rather a peer, a fellow officer, if you will.
In the latter scene Dean is not bequeathing his own quest upon Sam but he is finally acknowledging that Sam is an adult. One of the largest communication problems that has existed between Sam and Dean is that they have contined to interact on a child-parent level rather than a brother-brother level. Last season when Dean was upset with Sam he resorted to commands and threats, much like a parent, so Sam responded by acting out and sneaking around, much like a surly teenager. In a sense, it's not that they're each other's greatest weakness but rather their immature relationship is.
With the passing of the Knife Dean is functionally saying that he and Sam are partners, utterly and completely. They're finally becoming the unit they always should have been.
I LOVE it when Show is both so subtly smart and so aware of its own canon and character development. It's taken four long years but Sam and Dean have finally matured, both individually and as a team. Thank goodness.
ETA 10/3/09: I just noticed that while Dean is in the same position in both scenes, Sam takes and remains in John's position. Which is yet another indication by Show that Sam is John's heir. While Dean has strived to fill John's parental role both before and after John's death, Sam has grown to take John's place.
Except they're not John, they're Sam and Dean, a fact which may prove to be the world's salvation, as it has almost proved to be the world's damnation. But that's a whole other post entirely.