My recommendation is that deleting tasks needs to be a little easier. I hate those "are you sure" dialog boxes but on the other hand you don't want users accidentally deleting things. So do this: Make the user "double click" on the trash can icon to delete a task. That solves the problem elegantly.
There's 2 glitches. When I added projects then deleted the projects, their names still show up on the very left most panel. When I click their names nothing happens and I can't get rid of them. I had to refresh the page to get rid of them. When I click on mobile and then click the back button I get a red error message saying "Error saving to server. Check Connection."
In Firefox 11 for Windows 7 when you click the task boxes (which highlight blue) underneath the project's name the boxes change height slightly which is really awkward behavior. Setting overflow:hidden on the elements seems to fix it very nicely.
Color wise I wish it wasn't so grey. Too much grey can make a page look too cloudy and gloomy. Lighten things up a bit.
In the right panel, the tabs [Schedule | Charts | Help | Task]. Change "Task" to "notes". Otherwise it's confusing. And make the help tab the last one and the Notes tab (Task tab) either the 1st or the 2nd tab. The font-size used for them is also too big, use a font-size that's 95% of their current size. Also the tabs need some room to breathe, give the #tabs ul a padding:0 0 0 4px;. Also give #tabs ul li a padding:5px 8px 4px 8px;
Also you should show order and priority. There are 3 main panels. The left (project names) the middle (task list), and the right (schedule, charts, help, task/notes). In your css make the following changes: In #content #rightContainer get rid of the border-left, and add box-shadow:-1px 0 2px #666; and in #content #leftContainer get rid of the border-right and add box-shadow:-1px 0 2px #666 inset; It'll look much better because now it gives the app a slight 3D layered look. You could also do it in the opposite way (giving #leftContainer a box-shadow:1px 0 2px #666;) to emphasize the left panel first (which I think is more visually correct). It's just a little harder to do it this way because you'll have to create an inset box shadow for the right panel but because it uses tabs (with different divs on top) the inset box shadow doesn't show up unless you give ALL of the divs inside of it a box-shadow:1px 0 2px #666 inset; as well.
The button-links up at the very top (Sign up, Sign in, Feedback/help, mobile) should NOT be buttons but instead just links that are underlined. Get rid of the single "sign up" link and instead change "Sign up to save your tasks" to "Sign Up to change your tasks, it's free." and make the "Sign Up" part an underlined link. Change "sign in" to "Log In". And change "header nav's" padding to padding:2px 10px 0 0;
Make the changes and see if you like them.