I agree that this would be really useful, especially for the "plan to watch" section of the watchlist. I often want to see quickly which of my "plan to watch" shows will air the soonest, but this isn't possible without clicking on each show individually. If the Year column were replaced with Air Date, it would allow you to sort by Air Date, and it would make the "plan to watch" section more useful.
On a similar note, a more complex suggestion would be to make watchlist columns customizable per user. That is, each user could modify the columns (choosing from all available details for a show) to choose the ones that they wanted to appear in their watchlist. This would make the watchlist more useful for each user. To start, there would be default columns so that users don't have to modify anything if they don't want to, but those that want to can. And there would need to be certain columns ("title", for example) that couldn't be removed.
