Drupal has a wonderful form processing subsystem, especially in 5.x.

Now, hook_validate only gets invoked after pressing the submit button. But the form items which are rejected by hook_validate, such as required fields, don't provide good visual feedback to the user. There are a lot of average users asking why the form remains the same after pressing submit.

If validation could be done via Ajax, the fields which have errors (set by form_set_error) should give better visual feedback to the user which is easily noticed.

ATM, Ajax form validating should be compatible with 5.x, which means no more API changes.