Awesome project (hi Andrey!). Question: Why is a browser plugin required to access DApps like this? I don't immediately see a clear discussion of this on the web.
You don't need a plugin to view the DApp. If you log in without one you'll see a version pulled from a gateway.
In order to interact with the DApp (buy or publish an ad), you can use a plugin or a wallet app like Mist in order to be able to send transactions to the smart contract. You potentially do it manually by interacting with the contract through a ethereum console and sending the right transaction to the right address, but the browser plugin / app makes the process seamless.
Could also be a manual copy/paste of the contract address, ether and gas amount to your native wallet app, or a QR code to take a picture of with a mobile wallet.
Metamask (the browser plugin) is one - albeit a bit clunky in terms of UX - way of reducing the friction of doing web based ethereum payments, separated from your main wallet
In order to interact with the DApp (buy or publish an ad), you can use a plugin or a wallet app like Mist in order to be able to send transactions to the smart contract. You potentially do it manually by interacting with the contract through a ethereum console and sending the right transaction to the right address, but the browser plugin / app makes the process seamless.