For example user have Options page with long form in it. The most popular options are often on the top of the form, so user is changing them and have to scroll to the bottom of the form to hit "Submit" button. Imagine if user have to change something often and each time user should scroll to the bottom of the page to submit the form. IMHO it would be better for users to put "Submit" button above the form and they would scroll less. Example for testing two different types of forms on jsfiddle.
Sometimes long forms are splitted into tabs for making the form easier to use. But still it would be better to place "Submit" button above the tabs, because the height of tabs are different and the position on "Submit" button will not be the same.
Advantages of placing "Submit" button on the top:
- "Submit" button is always on the same place no matter of how long form is
- if the most frequently used options of the form are placed on the top part of form (as it should be) than in most cases "Submit" button will be visible for user without the scroll
Is there any disadvantages in placing "Submit" button on the top of the long form?
Possible compromise: maybe it would be good for users to put two "Submit" buttons in the long forms (one on the top of the form, another - at the bottom).
P.S. In the login or registration or survey forms "Submit" button should be in the bottom of the form because user will fill the text-inputs one-by-one and in the end user will hit "Submit" button. It would not be useful to place "Submit" button on the top of the form in these (or similar) cases.