Bitcoin was perhaps the worst possible example you could have chosen... The implementation was the opposite of mediocre; its code predicted and solved implementation problems that no one realized were problems, except Satoshi. And on a technical level, the client has only experienced one problem that had to be rolled back. There are few other implementations of anything that have that kind of track record.

A better example of a mediocre implementation of something would be the original Quake engine. Ha, got you, just kidding. Actually, I can't think of anything that's made "a lot of impact" while also being both a mediocre implementation and very difficult on a technical level. Even Minecraft wasn't mediocre. Difficulty and mediocrity seem to be opposites, the way magnetic poles are.