In Web forms, labels provide the questions that forms ask people. Input fields give people a way to answer those questions. Neither of these items, however, actually lets people complete a form. That singular responsibility rests with actions and actions are most commonly presented to users as buttons. So it's not surprising buttons get a lot of attention:
The use of buttons in web forms by Gabriel Svennerberg, September 2008
"Action buttons exists at the bottom of almost every web form. By gathering information from a few of the great minds in the field of web usability and also from my own experiences, I’ve tried to come up with a set of best practices on how to design them efficiently."
Buttons on Forms - where to put them, and what to call them by Caroline Jarrett, September 2008
"I’d love to tell you: put OK on the left. Or on the right. Or something else that’s easy to say and easy to remember. Like so much in forms, the simple answer isn’t really appropriate."
Previous and Next Actions in Web Forms by Luke Wroblewski, September 2008
"The quintessential question underlying this debate is: can an action which leads people to the previous step of a process be placed to the right of an action that leads users to the next step of a process?"
OK–Cancel or Cancel–OK? by Jakob Nielsen, May 2008
"Should the OK button come before or after the Cancel button? Following platform conventions is more important than suboptimizing an individual dialog box."
OK and Cancel Buttons: What's the Right Order? by Tom Tullis, March 2008
"There's some debate among web designers, usability people, and other geeks who think about these kinds of things, about the correct order for "OK" and "Cancel" (or similar) buttons in a web application. I decided to conduct an online survey of usability and user experience professionals to see what order they think is best."
Primary & Secondary Actions in Web Forms by Luke Wroblewski, August 2007
"While the primary goal of most Web form designs is to get people through a form as quickly and painlessly as possible, there are situations where slowing people down is advisable. When choosing between primary and secondary actions, visual distinctions are a useful method for helping people make good choices."