I suppose you don't have to be a beginner to take advantage of this, but newbies will surely appreciate seeing changes appear live. Mozilla Thimble allows a split screen, one side for HTML code, the other a preview that runs the code.
I hate HTML. I hate it with the power of a thousand flaming suns shining through a thousand magnifying glasses. There aren't enough words to describe my disgust, so this is perfect for someone like me, the occasional user. Super advanced users won't need it, but they have their own awesome tools. This is just for the normal folks, the dabblers.
Even current users are noobs to Google Drive, it hasn't been around that long. However, a major improvement is on the way with online syncing. Google Docs is so close to perfect that its few drawbacks are glaring issues. This is the last major one (besides their stubborn refusal to allow us to control appearance) to keep them from being The Most Awesome Thing Ever Ever Ever. Details aren't widely available, but it seems if you leave connection, you can continue to work, and as soon as connection is available your changes are uploaded. This would make two people in bad connection a bit like dueling doc editing, but for 99% of users this is a great thing.
All around, I'm happy enough to give Thimble a shot for my projects, and I hope my love affair with Google Docs is eternal. I'd love to hear back from any readers who want to speak up. Everyone's needs are different, if you have found a flaw or glowing positive, let us know in the comments!