Character Development:
Season 1 Dean: Charismatic, slightly broody, and aggressively straight. Could literally be a YA love interest.
Dean now: Grumpy, awkward bisexual nerd who fanboys over male wrestlers, is usually wearing a fuzzy bathrobe, and spends most of his time sad about the bae.
Season 1 Sam: Overachieving yuppie lawyer with no time for Dean's bullshit.
Sam now: Long suffering, traumatized moose person with no time for Dean's bullshit.
Season 4 Cas: Morally ambiguous, slightly bitchy, and generally terrifying force of nature to be treated with the utmost fear and respect.
Cas now: Cinnamon bun legit sweeter than a thousand sugar cubes. Spends most of his time being bullied and shoved around by literally everyone else. Less intimidating than an actual kitten.