Edit in JSFiddle

/* 
 * Disable submit button until password entered.
 */


(function () {

    var form = document.getElementById('loginform');
    var password = document.getElementById('login-password');
    var submit = document.getElementById('submit');

    var submitted = false;

    submit.disabled = true;
    submit.className = 'btn btn-primary';

    addEvent(password, 'input', function (e) {
        var target = e.target || e.srcElement;
        submit.disabled = submitted || !target.value;
        submit.className = (!target.value || submitted) ? 'btn btn-primary' : 'btn btn-success';
    });


}());

// Helper function to add an event listener
function addEvent(el, event, callback) {
    if ('addEventListener' in el) { // If addEventListener works
        el.addEventListener(event, callback, false); // Use it
    } else { // Otherwise
        el['e' + event + callback] = callback; // CreateIE fallback
        el[event + callback] = function () {
            el['e' + event + callback](window.event);
        };
        el.attachEvent('on' + event, el[event + callback]);
    }
}

// Helper function to remove an event listener
function removeEvent(el, event, callback) {
    if ('removeEventListener' in el) { // If removeEventListener works
        el.removeEventListener(event, callback, false); // Use it 
    } else { // Otherwise
        el.detachEvent('on' + event, el[event + callback]); // Create IE fallback
        el[event + callback] = null;
        el['e' + event + callback] = null;
    }
}

<div class="container">
    <form id="loginform" class="form-horizontal" role="form" name="form1" method="post">
        <input id="login-username" type="text" class="form-control" name="login" placeholder="username">
        <label for="login-password"></label>
        <input id="login-password" type="password" class="form-control" name="password" placeholder="password">
        <input type="submit" value="Login" id="submit" name="Submit" class="btn btn-success" />
    </form>
</div>

<style> #submit.disabled:hover {
    background-color: #999;
    text-shadow: none;
    box-shadow: none;
    cursor: not-allowed;
}
</style>