i think that implementing this without privacy options (like, being able to restrict who can send you stuff) is bad for sure, same goes for @ing people. after all, we know from experience that what can be abused will be abused
doing some Personal Opinion posting here:
"hey, look at this post" adds a lot of room for bad behavior. At least, that's my kneejerk reaction.
this seems like a nice compromise between not having private chats and being able to send people stuff on-site
something that really bothered me on the bird site is that when people wanted to share a cool tweet, they tented to mention users on the replies of said cool tweet instead of sharing it through the direct messages. so if your tweet got popular enough, your notifications would be flooded with these mentions, and that gets annoying pretty fast