Why do different pasta shapes taste different?
Different forms of pasta are going to have different surface-to-mass ratios, which will affect how the pasta cooks; the cross-section may affect how much starch gets rubbed off as the pasta boils.  (I can only assume that more's going to stay in a spiral or a tube than will on something that can rub up against other pasta)

But I'm going to assume that you're not eating the pasta plain ... and the different shapes will hold sauce differently, and that can be quite significant.

The other thing that Tom Gullen mentioned is variation in cooking -- I personally avoid capellini (aka angel hair), because I've had it overcooked so many times ... overcooked pasta is disgusting, in my opinion.