For this task (form-based text entry) a browser is great. You don't need anything that being a desktop app will give you (speed, flexibility)
There are draw-backs to being a web-application, such as..
It's a web-page. There are things you just cannot (easily) do
You cannot easily map the ctrl+j key to do something. For example: Google Spreadsheet tries to map keyboard shortcuts and works most of the time, sometimes the browsers default handling of the shortcut takes over..
You cannot make Growl alerts (An OS X notification framework). You cannot access the filesystem. It's difficult to allow access while offline.
Javascript is very CPU-heavy.
Try resizing a Google Spreadsheet document, or load a page on Digg (a very javascript heavy site) - the browsers CPU usage will be at 100% for a while.. Doing the same in a native desktop application is trivial
When you perform upgrades, you force them on all your users.
With a desktop application, they have the choice of not upgrading. For example, I didn't like one of the Google Reader upgrades, but I was stuck. Using NetNewsWire (a desktop application), if I don't like a change in the newest version, I can quite easily keep using this one (or try it, and downgrade)
You web-server must be accessible at all times, for ever
If the server disappears, your users have no recourse. The application is gone. If it's down for 10 minutes, they cannot use it.
With your application, while I'm not too sure what it is, none of the above seems like its going to be an issue.
"It's a web -page": Forms and dialogue boxes are easy to do in HTML and javascript (or even using server-side scripting, for example <?php if($_POST["email"] ==""){echo("Are you sure you want to continue?); ?>
)
"Javascript is very CPU-heavy": Doesn't sound like your application will require any Javascript (maybe some client-side input-validation when the user clicks "Submit", to warn them about any input errors?)
"Forced upgrades": I imagine this may be desirable, as you wouldn't want users inputing data in the old way.
"Server must be accessible": Could be an issue, but I don't think it'll be a large one.. Say you want to store all the users data in a central database, this issue becomes inescapable anyway - keeping a web and database server running isn't much more work than only a database (for the GUIs to connect to)
Also, you get the benefits others have posted - you develop it once, and it runs identically on every operating system that can run a sane browser.