Hey Zeeawk,
I am using it to create a super web app. I can not show anything until it is released. Basically I have been doing all the stuff you mentioned. Think of unity as Flash on Steroids. You can literally do anything you want. Its not like HTML5 and CSS3 where you can only do what the languages were intended to do. Here rather there is no limitations except that of yourself and your own understanding. We do have a few limitations imposed on us from the sand box just like Flash does also. So no file uploading in the Unity Webplayer the sandbox in firefox, chrome, iexporler, opera, and safari will not allow it.
The documentation is kind of thin with regards to the direction of Web Development with Unity. They have a set of WWW classes that lets you submit forms, download images, videos, etc. The interface runs faster than anything else out there. It is extremely fast. NGUI lets you minimize your images required to make an awesome looking interface really small which saves a lot on the number of images required to make your interface. Its way more efficient. You will be doing some digging in the API if you are not already very well versed with Unities API. Its a very good API with a ton of capabilities and a lot of good documentation. If you are not familiar with it you could try using Playmaker for some of it.
I will probably be releasing some new asset development in the asset store in about a month that I have already developed that will make creating web applications much easier in Unity. Of course you can do database backends with Unity. NGUI even has Json handling along with field cleansing before you submit the data back server side. Note making things scroll and edit anywhere you want in the text can be more difficult than you imagine. Also there is an HTML asset for NGUI in the Unity Asset Store.
Oh yeah you can not use Google Maps in the Web Player google forgot to make the exception for it for Unity like they did for Flash. They simply just do not have the file crossdomain.xml you will need and you will be blocked by security. You will need to use alternate mapping services if you want that.
Cheers,
Nate Nesler