Executing what? The word execution in the startup world refers to everything you do to build a startup, including cloning the competition. If you're talking about a specific type of startup, you need to identify what's hard beyond simply "execution" since that's a given.
Probably sales, getting people to buy the ads.
I'm referring to executing the Groupon "business model" which is what this article is really about. It doesn't take much "execution" to clone and create a landing page for subscribers. But thank you for your insightful knowledge into the meaning of the word "execution" in the startup world.