• Web browsers provide a really convenient, if limited, client interface. They are also non-standard. IE, as an example, is non-standard within itself. (IE 5 and IE 6 render the same thing differently.) But, it does provide a much quicker time-to-market with a standard look-and-feel.
• The Web's stateless nature is inherently unsuited to desktop-style applications. While the actions of a Word or Excel are stateless, what they act upon is their state. Now, if browsers were to provide more information to the server concerning state and allow themselves to be more configurable (removing the back button or allowing overloading of the back-button's functionality), that would be helpful. But, can you imagine every browser in the world doing that? In a standard fashion?

Ultimately, them ore control you want over how your users work, the more control you need over how they interface with you. In my mind, the best of both worlds is to provide a client browser to your users, preferably written using something like Perl/Python/Tk. That gives you most of the OS-independence and allows you the server-side benefits. This is the model that most of the MMRPGs use, and it works really well.