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Sure. Here's one: I can drag and drop an attachment for an email right onto it. No can do in gmail.

If you actually look at how people use email you will find quite a bit of diversity. However, one of the common threads in email usage is task management. This highlights one of the biggest areas of deficiency in Gmail. Take the lack of calendar integration as one example.

It's more of a long-tail effect. Outlook has a lot of features, and it's a bit bloaty in the sense that they could be better organized but having supported people using outlook for a couple of years I can say that people find the Way They Like To Do Things and it's not always the same as everyone else. People find the feature they like and use it. Delegate configurations are very common executives and their assistants. A friend of mine used the notes feature all the time.