GameInformer reports that Sony has implemented a system on PlayStation 4 which will tell players exactly how rare the trophies they've unlocked are.
When you unlock a trophy, head over to the trophy screen and you'll see a rating that identifies an unlock as common, uncommon, rare, or ultra rare.
Determining how rare a trophy is comes down to player activity. The more players who unlock a trophy the more common it will be become so presumably the trophy for completing a tutorial will be more common than one for finishing a game.
Presumably, this means trophies could also become more rare as additional gamers start playing the titles.
Trophies for PS VIta, PS3. and PS4 will be displayed on a single screen and GameInformer reports that Sony has eliminated the slight lag that exists between player action and trophy unlock.
You can get a better idea of trophy unlocks on PS4 here – another change is that popups now go on the top left rather that top right.