A group of retired players filed suit against the NFL because, quite simply, this labor deal may impact them, too. After one year without a CBA, all of their benefits get terminated. They make a pretty good living out of the pot of money that the NFL grosses each year. They're worried.
On another note, there's some growing concern about the upcoming draft. The NFL says it will go on, and apparently a few prospect players have chosen to appear at the draft and glad hand with their future employers. But. Remember that the draft is governed by the CBA, and there's no guarantee that the prospects will simply agree to go along with it; nor is there a guarantee that the owners will behave according to the "rules" they have laid out.
Plus, there's this April 6th court date that may throw a wrench into everything.
And finally, there are the players themselves. Now the trade association (not the union) will have to decide how to handle the fact that the players will undoubtedly be out of shape when they do report, that the rookies and players who have been on the street for some time will have had no or little game experience at this speed and intensity. And as a result, there will be more injuries. It will be interesting to see if that gets them to consider an expanded roster - beyond what they previously asked for.