Why would I want to write a web app with lots of processing server-side?
To me, writing the program client-side is a huge advantage because it takes away as much server load as possible because it only has to send data to the client with minimal processing.
I see very little on writing web-apps besides writing it server-side and treating client-side as only a view. Why would I ever want to do this? The only advantage I see is that I can write in whatever language I want (http://www.paulgraham.com/avg.html).
Best Answer
There are two major issues.
The first is easy--you usually don't know what sort of resources are available on the client side. If it requires 1.5GB to process something, can you really push that onto an unknown client browser (IE, Safari, Opera, Firefox, etc.) on an unknown client platform? Will the client appreciate his system dogging when you overwhelm it?
The second is more architectural--what layers do you want to expose to the outside world? Most would agree that it's incredibly risky to expose your data layer. How about your service layer? Do you really want to deliver that logic out there? If you do, are you also exposing the entry points to your data layer? If you keep the service layer server side, then what's left? The UI, right? See reason 1 about for considerations on how much of that lives on the server and how much on the client.