I think the replayability is evident from the first playthrough, and I think people will definitely want to try out all kinds of options, whether to do "better" through the winter or summer, see if they can get more hints as to whether or not Breden really is the traitor, check out the "powers" available from having a different skill, or just working on the achievements.
I think what makes XoR especially successful in this regard is the simplicity of the stats you created for the underlying system. There are only 3 main stats, each with very clear and specific perks. You simply choose your strength and weakness once, then the rest of the game reflects your choice back at you. Each time the MC fails at something it's obvious which other skill was needed to make it work, and that encourages players to try a new character to see what would happen if they could succeed.
Add in the genuinely difficult decisions to make, as well as the beautifully written "flavor" text, and the million-and-one tiny things you added that make every playthrough a little different and I have a hard time imagining the person who would play it only once and never think "I wonder...".