One thing that becomes a struggle with landing pages is making sure they look good in IE, Firefox, and Chrome. 30% of our customer-base still uses IE8 as their default browser which causes problems for us because Marketo doesn't play well with IE8.
I use Firefox as my default web browser for building and using the Marketo application, but find I have to make adjustments multiple times and preview the landing page in 3 different browsers before pushing them live.
It would be great to have a preview option in the landing page designer that would allow you to view your landing pages in different browsers without having to open new browser windows to review your page.