Making it easy to both join and leave
Some web applications makes it easy for you to join and even leave and uses this to help lower the entrance barrier of getting the user started with a service. Blogger.com and wordpress.com for instance makes it dead easy to import data from your old blog and even export your content once you decide to leave.
Basecamp is easy to integrate with your own system
The project management application from 37signals provides both an RSS feed of your projects’ activities, and an extensive API that lets you integrate the application with your own systems making you both read and write as much as you want. The openness from 37signals has resulted in several iPhone applications and has led other web app providers (30+) to offer seamless integration with Basecamp – all of which only makes Basecamp a stronger product.
When excitement is part of the equation
In January, I bought a ski vacation from Nortlander. There was nothing special about it until two days before my vacation started. This was when I received an email from Nortlander telling me about the snow conditions, the sunny weather forecast in the upcoming week, and that they looked forward for taking care of us. Even though it was easy to spot that it was a mass mail, my already high level of excitement rose even higher and was poured into my view of Nortlander as a company as well.
There are so many ways you can improve the whole experience
There are plenty of ways to improve the experience of your online product, that does not have to do with the web experience and application itself. When you are competing with many similar products, improving and focusing on neglected parts of the experience might just give you the edge you need.

GUI Redesign
28 Aug, 2009
That is really what it means being up ahead of the competition.
Web 2.0 Design
28 Aug, 2009
There is definitely something about web 2.0 applications