Beyond the obvious differences in flavor, do these two tend to be interchangable? The consistencies and composition seem close enough to me to make an acceptable substitute, but is it likely to give a significantly different end product with certain types of preparations?
Nutella is over half sugar by weight; peanut butter might be more like 5-10% depending on brand/style. That already sounds like enough that it won't make a good one-for-one substitution in most things, especially anything that's not already pretty sweet.
It probably also means that it'll have a slightly different resulting texture; nutella is already a bit less thick (probably again affected by the sugar), and I believe this is pretty obvious when heated, though it's been a while since I tried.
I just used nutella in some mini chocolate chip muffins that called for a banana, honey and peanut butter. The texture was not the same. I would not do it again.