For starters, PlayStation (PS4 and PS5) players will get access to certain cosmetics for each hero – both starter heroes and post-launch heroes – 30 days before Xbox and PC. This includes one legendary skin and emote, one epic takedown animation, and one nameplate per hero.
In a similar vein, PlayStation Plus subscribers will receive a free rare skin, a nameplate, and 100 in-game credits each time a post-launch hero is released. This includes a launch bundle for Ms. Marvel.
All of this was announced on the heels of Sony and Crystal Dynamics' Spider-Man arrangement, which will lock the web-swinging hero to PlayStation platforms when he arrives in 2021. As of now, it doesn't look like Spider-Man will ever come to other platforms, which suggests his role in the game's story will be minimized to prevent continuity issues.
Technically, none of the additional content revealed today is outright exclusive. All of the cosmetics outlined above will be available on Xbox and PC a month after their PS4 release, and while Xbox and PC owners won't receive them for free, they will be able to earn (or purchase) the skins and nameplates distributed through PS Plus. Nevertheless, between Spider-Man and this suite of trinkets, it's clear that Marvel's Avengers is pretty noticeably skewed toward PlayStation.
PS4 players will also get early access to the Marvel's Avengers beta, which will open in blocks throughout August. You can find a full schedule here.