Gmail Features

Search/filter on arbitrary headers
It would be useful if you could search on arbitrary headers as well, like List-Id, Delivered-To etc.
Force different subjects into single conversation
It would be useful to force different worded subject into a single conversation

Search attachments (PDF, Office docs etc.)
The ability to search through email attachments

Create email aliases
The ability to create aliases (different account names)

Customizable keyboard shortcuts
It's an ability to make different actions to different keystrokes, or keystroke combinations.

Associate contact with label
Most Gmail users may set up filters to label mail from certain people automatically.

Delete attachments only
Delete attachments only, not the entire email.

Manually break up conversations
In some cases it could be useful to break up a conversation into separate emails, or several conversations.

Virtual search folders/labels
Virtual folders/labels, or stored searches, if you will.

More filter actions
This filter mechanism could benefit from adding ability to specify additional actions.

GPG/PGP support

Encrypt and sign emails

Read receipts
You can get a notification when the recipient has read your email

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Compare Gmail and Yahoo Mail
why use Gmail ???