Regarding Coulson, and I acknowledge mine might be a minority opinion, but I didn't feel that his return in Agents of SHIELD 'cheapened' his death or what it meant in Avengers at all. I think it would be different if it had been revealed that the medics had indeed been able to save him then and there and it was a case of Fury lying over the comms, but that's not what happened. He *did* die, and he was dead for days (at the very least) before they put his body through an experimental procedure (one that, it turns out, Coulson himself recommended NOT be put to use on anyone, ever) that brought him back.
Personally, I don't mind people being brought back from the dead so long as there is some type of ramification to being brought back. If they're brought back and everything is just sunshine and roses and no consequences to the action... then *that* I would have an objection to. But there being consequences (like there was for Coulson, with the side effects of the TAHITI project and all that came with it), I think that works... it still gives what they went through weight and meaning.
On the subject of Quicksilver, count me among those that think he should be brought back in some way. I think it was an extremely stupid idea of Joss to kill him off in the first place as (IMO) he seemed to do it because he felt having someone die was the only way to show 'stakes'. It's not. You can have stakes without resorting to character death.
Just my opinion... apologies if it didn't make sense.