This post from Brendan Gahan really nails some key points with how to get the best out of YouTube, and the audience you want to reach. One point really sticks out though for me, the use of gadgets or microsites embedded into a YouTube channel. It doesn't make any sense, there's a website for that.

Use YouTube for videos, not a website jammed into a separate tab on the channel.

Oh yeah, it doesn't work on mobiles or tablets either.

Lot’s of brands use channel gadgets (here’s an example of one). These gadgets are apps in an iframe on the Youtube channel. It interrupts the user experience and forces viewers to interact with the brand in a manner that they’re unfamiliar with. As a result there is a ton of dropoff (50% fewer viewers convert to subscribers, and there are 15% fewer shares than brand channels without gadgets).