While it's true that you're more likely to have your lost belongings returned to you here than in many other countries, it certainly isn't guaranteed. I learned this the hard way. I once left my DS along with about ten games on the bullet train once (about 50,000 yen worth of merchandise). Despite hours of searching and calling, I never got it back. I'm sure I made some otaku very happy that day...
As for people dropping things, think twice before you pick the article up and return it to the owner, especially if it's a wallet. Some people deliberately drop wallets in an effort to ensnare foreign Good Samaritans. While I don't think this scam is widespread, I directly witnessed it happen to my friend a few years ago. What happens is they drop the wallet in front of you, and when you predictably pick it up to return it to them, they claim that money is missing from it and that you must've stolen it. If you don't pay them, they go up to the nearest police box and get them involved--and we all know how that ends, especially if the foreigner speaks minimal Japanese. Luckily, in my friend's case, he ended up trying to hand it off to the wrong person and unintentionally made things awkward by getting random bystanders involved, so the girl who "dropped" the wallet abandoned the attempt and probably went off to look for another mark.
Again, most likely a rare occurrence, but just something to keep in the back of your mind.