Pick an application category and start building a product with just enough features to be useful for a newbie. One that might last a non-demanding user forever.
You can see the need for this if you're one of the people who was just switched to the new shell for Gmail. All of a sudden all the stuff that faded into the background comes to the front, demanding attention. Because you have to wade through all that to find the commands you need. The ones you used to be able to find with just the intelligence at the base of your spine. No need to involve higher brain function. You can see the need for a bare-bones email interface because you're looking at the bloat of a mature product that's been hitched up with all kinds of sales jobs. It has to sell all the other Google stuff. But all I want is email. I thought maybe I'd switch to Yahoo Mail, but it's worse. It actually has the new style ads with obnoxious and deformed people dancing and looking weird, to draw your attention to their message, selling whatever it is those ads sell.
Maybe I'm missing some stuff. This would be the subject of debates and conferences. There might even be a professional standard set of features that must be in an email client in order for it to get the "minimal" seal of approval.
In 2012 there should be a healthy set of minimal web-based email clients. I don't think there's a single one.
Actually there was an early word processor called Volkswriter. It was a response to feature bloat in early WP apps.