> What keeps me from running my blog from home?

Depending on the contract it might not be allowed to run public network services from your home network.

I had a friend doing that and once his site got popular the ISP called (or sent a letter? don't remember anymore) with "take this 10x more expensive corporate contract or we will block all this traffic".

In general why the ISPs don't want you to do that (in addition to way more expensive corporate rates) is the risk of someone DDoS that site which could cause issues to large parts of their domestic customers (and depending on the country be liable to compensate those customers for not providing a service they paid for)