Alex Payne has a great criticism of both Web and desktop email application in his post The Problem With Email Clients which accurately captures some of the frustrations I've had with both classes of email clients. The entire post is a must read if you've ever thought about how email can be improved. Some key passages from his post are 

Anyone who’s given Gmail a fair shake will quickly find conversations indispensable. Going back to any other email client is agonizing and disorienting, like being knocked around and dumped out of the back of a pickup on the outskirts of a strange town. In desktop email clients, new messages arrive completely bereft of context. The only way to orient yourself is to either remember what the conversation was about or read through the mess of quoted text that may or may not be present at the bottom of the message, depending on what kind of email client or prefences the sender has. You could try searching to re-orient yourself, but good luck with that in Outlook or Mail.app.

With conversations, Google has offered the only advancement in the information architecture of email clients in decades. Apple, on the other hand, has given us basically bupkiss, rendering Neven’s defense a bit silly.
…
This is probably heresy coming from a web application developer, but I think web applications are mostly ghastly. I hate using them. When I’m faced with a computing problem, I want to solve it with a polished, stable, native application for my operating system that looks and feels like it belongs on my computer. I don’t believe in Rich Internet Applications — they’re a boogeyman that I keep hoping will disappear.

The problem with Gmail is that it could be a “real” application. While its conversations and search-that-actually-works are (sadly) innovative, they’re not impossible to implement as part of a platform-native email client. I enjoy using Gmail, but I’d enjoy it even more if it obeyed the rules of my operating system, not the rules of the web. The web has a lot to offer certain types of problems, but I’m not convinced that email is one of them.
…
The problem with desktop email clients is that they’re not webmail. The problem with webmail is that it’s not a desktop email client.

I think Alex is on to something here. I prefer to read read my Hotmail account in Outlook via the Microsoft Outlook Connector because I'd rather read my email in a desktop app than in a Web app trying to look like a desktop app. On the other hand, I read my RSS feeds in RSS Bandit even though Outlook is an extremely popular RSS reader because I'm dissatisfied with how Outlook presents messages and feel an app I wrote myself does it more effectively.

Clearly there is room for a revolution here.

Now Playing: R.E.M. - Everybody Hurts