Minecraft 1.7 did not include any major changes. They just rewrote about 50% of the source code.
Nah, not a lot.
And they're doing it EXACTLY so they can make a mod store.
Now, the only reason you cannot pay for Railcraft is because Mojang included a clause stating that you cannot sell modifications or derivative works based on Minecraft's source code. They ARE working on a mod store and API... since 2011. I believe it will be coming in the next year or so, Mojang claims January but I stopped believing them long ago.
About modifications, I started work on a way to improve mod distribution from servers to clients: AsieLauncher. It automatically generates a launcher, a list of modifications and a list of changes between modpacks. It supports delta updating, all MC versions from 1.2.3 to 1.7.2, optional mods and a lot of other things.
The plan was to add support for automatic mod updating and dependency tracking, but the problem is mod authors aren't often willing to participate, due to stubbornness or just plain assholery or laziness. That's why such projects rarely take off.