I like commencement speeches. I think they come at a time when the graduates in the audience believe they are thinking about the future, but are really only thinking marginally further ahead. Rather than an immediate focus on final exams, they may be dreaming about the next four years of college, or maybe planning for their first steps in an entry-level position after college. In the scheme of things, that's still a pretty tight focus.
A good commencement speech should inspire a bigger, broader, more encompassing mindset. It should shatter the short-term focus on "what to do" and recenter the audience on "how to do". It should also hold the attention of antsy graduates and engage them at a personal level.
I don't give Conan's speech at Dartmouth my highest marks like many in the press have done, but it is pretty good. Here's a short video of highlights that focuses more on his humorous lines than on his wisdom. The entire speech is also available on YouTube if you wish to see it.