I don't feel that the show has downplayed or mistreated Sam at all.
Sam wasn't selfish to want a different life for himself at all (because that is normal -- growing up means taking what your upbringing has given you and applying it to your own choices, making your own life), but I also don't think Dean was selfish to want him to stay -- Sam is all he has.
I think it's also telling, as Ailleann points out, that Dean never guilted him into staying. Even in the pilot, the decision to return to that life is Sam's, and it's horrible it took Jess's death to provoke it, but it also gave Sam some insight into how John and Dean must have felt when Mary died.
I also think it's always been made clear that Sam doesn't feel what they do is unworthy at all -- his empathy for the victims is often more poignant than Dean's. His focus may have been sharper at times -- don't take the minor cases, focus on finding John and the demon -- but he knows that what they do is something most people can't, and that a lot of people need them.
All of the storylines you say, Morgana, that the show has dropped for Sam don't seem dropped to me -- as Plei points out, they've evolved as the reasons behind his destiny have. Maybe death, however brief, fucked with Sam's psychic visions and telekinetic powers, but either way, I don't the lack of them now makes Sam any less important to the plot.
I think it's also important what both boys have learned as the series has progressed. At the start, it was the younger son wanting normal and the older son blindly loyal to the family tradition -- I think the show has gone to great lengths to make it clear both that Sam has been able to see some of the reasons for John's choices, and that Dean has learned how much damage that upbringing did to both of them, despite whatever good they do when they're saving people. Sam is the one, to me, who remains the least damaged, despite his demonic blood blah blah -- he's still fighting, still willing to believe in them and what they do, and determined to rectify his own mistakes.
Neither of them is perfect in my eyes, neither of them is more right than the other (most of the time), and I love them both despite their flaws. If anything, I wish Kripke hadn't decided to make their destiny predestined -- I think it would have been interesting if their choices were solely based on their upbringing, their perspectives, the differences between them.
That said, I like the way they're handling their decisions to fight that destiny, too.