The original is one of my favorite games of all time. This is really just an extension of the original.

The overworld is exactly the same. The only changes seem to be in those dungeons where you get a dungeon map. These dungeons are a fair bit more challenging. Their overall geometry is mostly the same, but you traverse the dungeons in a totally different order, with different puzzles. For example, you might get the dungeon's item (e.g. hammer) right away, and then have a bunch more puzzles involving the hammer's use.

It's really just the same game with new dungeons. If, like me, you've played OoT half a dozen times, you will certainly enjoy playing through it again. If you've thought about playing again, this is a great reason to do so. If you didn't care for the original, you will not care for this one.

It's great for the fans, but I don't know that I would have bought it if it had been offered when the N64 was still kicking. It IS the best pre-order freebie I've ever heard of, though.

On the GCN, the graphics are somewhat updated, with higher resolutions, and maybe cleaned up textures. You're not going to be amazed by the improvement, but it certainly didn't suffer from the transition (other than small weirdness with the sky).