The pre-order page for Overwatch details what all you will get for your $40 in cash money. You get the game and all currently announced characters and maps. Now, if you bump it up to the $60 Origins Edition you then get the following.
• Hero skins
• Tracer Hero for Heroes of the Storm
• Baby Winston Pet for World of Warcraft
• Card Back design for Hearthstone
• Mercy's Wings for Diablo III
• In-game portraits for StarCraft II
• Tracer Hero for Heroes of the Storm
• Baby Winston Pet for World of Warcraft
• Card Back design for Hearthstone
• Mercy's Wings for Diablo III
• In-game portraits for StarCraft II
Yep, $20 more and most of it goes towards cosmetics for games that aren't Overwatch. Oh, and it seems as though pre-ordering does not even guarantee you a spot in the ongoing beta. Of course, charging $40 at a minimum for a game that has very similar free-to-play competition on top of including microtransactions seems like a super terrible idea.
There's also a $130 Collector's Edition. This comes with a soundtrack, a "visual sourcebook," and a Soldier: 76 statue. This is on top of the content in the Overwatch: Origins Edition.