The iOS 4 email client also threads messages, but you can turn off the function. If there are related messages (based on the subject line), you'll see a numeral to the right of the subject line indicating how many other messages are in the thread; tap it to see the related messages. This feaure can be quite useful when you need it, and it's unobtrusive when you don't.
Note that these email changes won't affect Lotus Notes users, since IBM provides its own email client (Lotus Notes Traveler) to handle message management and display. Ditto for the Good for Enterprise mobile management tool, which also uses a separate email client with its iPhone app. The email changes in iOS 4 described here affect only email accessed through the iOS device's built-in Mail app, not third-party mail apps.
Where iOS 4 Mail still falls short
Although these email changes are largely welcome, iOS 4 continues to omit key mail features. You still can't open Zipped file attachments, for example. You can open email attachments in other apps, but only if they use the Open In API originally developed for the iPad. To find out if your apps can open a particular kind of attachment, tap and hold on the attachment icon. A list of compatible apps will appear; note that if the sole option is Quick Look, you can view the attachment only in Mail's preview capability.
Several basic email capabilities you'd expect in a more desktop-like Mail app remain missing, including the ability to mark all or multiple messages as read, the ability to create folders in Mail, the ability to create and use group lists (it doesn't even see your synced Contacts app's groups), and the ability to set separate signatures per email account. Those are the basics; I'd also welcome junk mail filtering, message rules, and message flagging. Here's hoping for iOS 4.1.
Enhancements to security, calendar, notes, and Safari
IT admins will be happy to know that iOS no longer restricts user passwords for unlocking the device to numerals. Instead, they can now use real passwords. Of course, if you use Microsoft Exchange ActiveSync policies, you had the ability to require alphanumeric passwords in iPhone OS 3.x, but users could only opt for such passwords if an Exchange policy required it. Now, users can set up such a password themselves whether or not it's required by IT.
As a user of an iOS 4 device, you may notice flight departure times and other appointments showing up in your calendar. Apple has created an API to let app developers access the iOS Calendar app, so if you book a flight, it shows up in your calendar automatically. Of course, this functionality has to be enabled in each app. (Again, Lotus Notes users won't see this because IBM's Lotus Notes Traveler uses its own calendar app, not the built-in one from Apple.)
Any notes you create in the Notes app now sync to IMAP-based email accounts, not just Exchange and MobileMe ones, via a Notes folder. Keep in mind that these notes may not be editable in other email clients; it depends on the client's specific capabilities.