YouTube co-founder Jawed Karim on How YouTube Got Viral.

1) related video recommendations
2) one-click emailing to spam a friend about a video
3) more social networking and user interaction tools like video comments
4) an external video player

Re: #4, the external video player did have an amazing impact. In a matter of months, YouTube seemed to go from nowhere to everywhere due to that slick and easy to embed player.

buttons

A big reason why the external player was so effective: Play buttons are seductive. When people see one, they instantly know what it does and want to click it.

Similarly, Coudal’s technique of showing showing video toolbar buttons in its Jewelboxing ads is also a great way to attract clicks.

coudal player

Some more musings on YouTube’s interface:

YouTube's Interface: If You Build It, They Will Come

So, in a space with plenty of big players, but no real successes, how did start-up YouTube manage to get so big, so fast, and why was it successful where other big players were not? It’s the interface, stupid! While the technological and bandwidth barriers to getting video online easily have only just recently ebbed away, YouTube managed to be the first to take advantage of this new opportunity in a way that, quite simply, works.

Harmonization of the interface

Streamlining and harmonizing the interfaces people need to use to get to you makes good sense. YouTube offers a way for its users to search, navigate and mark favourites that each user knows how to do instinctively after the first few times. As Steven Johnson says in his book Interface Culture: “…knowledge becomes second nature to most users because it has a strong spatial component to it…” And so it becomes easier for people to find my videos on YouTube, because they don’t have to learn the user interface of my own website.

Worth noting: Chad Hurley, YouTube co-founder, comes from a design background (he started his career as a graphic designer and worked on PayPal’s logo and its user interface in his pre-YouTube days).