Traditional paper and pen go a far way over email in saying a "proper" Thank you. If you have occasion to send a Thank You, then someone when out of there way to do something kind for you. Therefore, a small bit of additional effort over a 30 second email is the most courteous way to reply.
I know "times are a changing" and I am not saying an e-mail Thank You is never appropriate, but should remain the exception, based on the situation, rather than the norm.
People still feel good about opening a lovely piece of stationary with a thoughtfully hand written note on it. Brings a smile to my face every time and is a welcome break from the junk mail and bills that otherwise fill our mailboxes!
I just had twin and I am inundated with Thank Yous to send. I remember sending a very carefully chosen gift to a friend recently and while they still sent a card, it was a mass production "Thanks for for the great gift" preprinted job and I was pretty offended. Neither my name, nor the gift was mentioned, really irked me. I wanted to be stroked a bit, I guess, and at least see my name mentioned. It was really a lovely gift.
Oh, well...I would have preferred an e-mail to that, I will say.