My primary concern is him saying "well, it's free speech" while letting people spread unfettered lies, because apathy is easy for someone with billions of dollars.
At least within the US, that misinformation was incredibly harmful in 2016, 2020 and over the course of COVID. Twitter already was doing the bare minimum to combat them.
More topically, how much less concrete would US support for Ukraine if Trump had been allowed a platform to spread Russian disinformation on the daily, rather than his second-hand, sporadic statements kicked out.