When I was a sophomore at Hopkins, my advisors (Steve Salzberg and Simon Kasif) warned me to read and study what I liked as an undergrad, because as a grad student there'd be less opportunity for breadth and for taking that Shakespeare: Plays to 1600 course you're really interested in.
Memind me sometime to quote you a conversation between the nanotech futurist Eric Drexler and Grant Fjermedal, published in the latter's 1986 book The Tomorrow Makers: A Brave New World of Living-Brain Machines.  The essence of the conversation was an apologia for Drexler's intent and desire to download his mind into an artificial body, in that he had about 1500 years of things he'd like to do - which sounds profoundly unimaginative if you ask me, but wotever.
(I suddenly recall Elle MacPherson's character, Janelle, discussing this with Ross Geller on Friends, and am now sad as I realize this scene probably gave the whole concept more publicity than all other documentary TV discussions of it combined. :-P)
Here's my take.
As a university professor in CS, I feel it is our responsibility to be frank and upfront with students and let them know if we see their efforts as leading nowhere fast. Having said that, I think the American university system has tremendous potential for inappropriately crushing the enthusiasm and spirits of young people who are actually very motivated and talented, but just haven't been given the chance (or have had personal issues get in the way).
I have had quite a few very good students and also a lot of mediocre and poor ones, and I often feel in awe of what really talented young people are capable of.  So, in short, if you are a student, take criticism in stride, and often with a grain of salt. The criticism of professors is sometimes useful, but we really should take care not to take too broad a blade to people's aspirations, and too often we do not. Don't let it get to you.
What was it that Dr. Einstein said?
Do not worry about your difficulties with mathematics; I assure you that mine are greater.
Certainly, I wouldn't have a Ph.D. in CS now if I'd listened to the first instructor to dump a bucket of cold water on me in my early undergrad days. :-)
Edit: Talk of The Nation: Science Friday - highlights from the Ig Nobel awards. Thanks, deire! My favorite is the Peace Prize, to Lal Bihari, founder of the International Society of Deead Epople.
 Here is the Amazon.com page for The Tomorrow Makers, and here is a bookseller that sells used editions.
 Case in point: courtesy of the web-sniffing skills of those wacky kids in cty_therapy, here is Daler Mehndi.