Steam provides not only a storefront with lots of features that devs can use, they also provide servers, backend support through different APIs and not to mention the insane amount of data users need each day. (Around 275 PETABYTES a day, or 100 exabytes a year) That kind of infrastructure is not cheap, and seeing as that was the industry standard (it's still what Google and apple get from their storefronts) I'd say that's pretty reasonable.

Also the 30% cut goes down to 25% after the first $10 million in revenue, then down to 20% after you reach $50 million. I do think they should put in a lower cut for independent developers who are releasing small games, until a certain revenue threshold is reached, but overall I would say it seems reasonable to me.