Spoilers ahead for the most recent episode of Game of Thrones.
Jon Snow isn't the first person to be resurrected in Westeros, and the old gods be willing, he won't be the last. But what do we know about resurrection in this world, and what might this particular one mean for our favorite Stark bastard? Even if the resurrected wights have no bearing here, Jon's return may not entirely be a good thing. As George R.R. Martin has said before, "My characters who come back from death are worse for wear. In some ways, they're not even the same characters before. The body may be moving, but some aspect of the spirit is changed or transformed, and they've lost something." In other words, he could come back wrong. This is how the showrunners and Kit Harington can get away with saying that Jon Snow wasn't coming back this season — whoever this is now, he may not be the Jon Snow you knew and loved.
He may not be entirely himself.
His personality could be altered. We'll let Beric Dondarrion, who has the most experience in these matters, explain what being brought back to life has done to his humanity: "Every time I come back, I'm a bit less," he says. "Pieces of you get chipped away." Beric, at least, had some empathy left, but that could be because Thoros of Myr would perform the ritual immediately after his death. Jon's body had some time to get cold, and that could affect his ability to feel. (A separate deceased character from the books, Lady Stoneheart, was left cold for three days before her resurrection, and she has, well, a stone heart as a result.)
He may not remember his past.
His memories, especially long-term memories, may be lost, even of loved ones. In the books, Beric elaborates. "Can I dwell on what I scarce remember? I held a castle on the Marches once, and there was a woman I was pledged to marry, but I could not find that castle today, nor tell you the color of that woman's hair. Who knighted me? What were my favorite foods? It all fades." Ygritte may finally be right — Jon could know nothing.
His wounds may never heal.
Beric Dondarrion doesn't become magically whole again with each resurrection. The eye he lost? Still gone. His scars? Still there. In the books, his temple remains caved in. As for Lady Stoneheart, her wounds remained open: "The flesh of her face clung in ragged strips from her eyes down to her jaw. Some of the rips were crusted with dried blood, but others gaped open to reveal the skull beneath." Jon was stabbed a number of times, and physically, his body may never fully recover. Any decay that has set in will be permanent, although with his luck, the cold has preserved him. Still, his physiology will be changed. In the books, Beric seems to no longer need to sleep or eat, "though from time to time, he took a cup of wine."
He might not be able to speak.
Ser Gregor Clegane, a.k.a. Ser Robert Strong, appears to be a mute monster in his resurrected form, although Qyburn explains that away as a vow of silence. Lady Stoneheart also can barely speak, croaking only a few words, but that might be because her throat was slashed.
He's released from his Night's Watch vows (and needs a successor).
When Jon Snow pledged his service to the Night's Watch in front of a heart tree, he swore these words: "Night gathers, and now my watch begins. It shall not end until my death." Well, a death is a death, and there's no apparent resurrection clause.
If he's no longer the Lord Commander, that means a new one will need to be selected, and considering the events of tonight's episode, Alliser Thorne is in no way assured of victory. Perhaps Eddison Tollett, a Snow loyalist, has a good shot at command?
He could return to the Starks and/or lead the North.
Jon's demonstrated time and again that he's been torn between his duty and his love for his family. This was why Aemon Targaryen revealed himself to Jon, telling him, "Love is the death of duty," and reminded him that there would be days that he would be forced to choose. Jon refused Stannis's offer to legitimize him and become Lord of Winterfell because of his vows, but if the vows no longer hold him back, it'll be easier for him to come to his family's aid, or take the North from the Boltons. That is, if he remembers his scattered siblings.
He could be the embodiment of a prophecy.
Melisandre thought Stannis was Azor Ahai, the Prince Who Was Promised — a champion who would be reborn to stand against the White Walkers and make the world anew. This was what her whole ceremony presenting Stannis with a burning sword was about: "After the long summer, the darkness will fall heavy on the world. Stars will bleed. The cold breath of winter will freeze the seas, and the dead shall rise in the North. In the ancient books, it is written that a warrior will draw a burning sword from the fire, and that sword shall be Lightbringer."
Of course, Beric Dondarrion had a flaming sword as well, and other red priests and priestesses have proclaimed Daenerys Targaryen to be this reborn hero. But now we have a new candidate in Jon Snow. In the books, Melisandre has seen this possibility in her flames, but she doesn't give it credence: "I pray for a glimpse of Azor Ahai, and R'hllor shows me only Snow." The red priestess had a bit of confirmation bias there, but Stannis is now dead and Snow is alive. Could Jon Snow be the Chosen One? Is there even such a thing? Or should we agree with Jaime Lannister when he says, "Fuck prophecy. Fuck fate"?
We, too, know nothing.