I am very very very pro rounded corners for clickable buttons. Rounded Corners is one of the main things that lets a user know a button is clickable. In fact roundedness was so important in UX that in the past before we had the border-radius: property in CSS we used an actual image made in photoshop as the button. Don't throw out the baby with the bathwater.
Websites used the native <button> and <input type="button"> so they got that native windows and mac button look. Square, beveled, 3d look, gray. Then to style them they would just use <a> links with images inside, now we're back to code with css.
