I was also frustrated when composing emails. If you have multiple email accounts, you always have to choose one before composing the message, even when you're working in a particular account. Other OSes assume it's the account whose emails you are reading or let you set the default account. As in other mobile OSes, the BlackBerry 10 OS lets you change the From account before sending an email -- as long as you swipe up at the top of the message to reveal the menu for doing so. It's not as intuitive as in iOS or Android.
The contacts lookup when addressing emails was often very slow. And after you type part of a person's name to initiate a lookup in your contacts list, the BlackBerry 10 OS leaves the partial entry you typed for you to delete. Why?
I experienced more text-entry blues. As you type, suggested words pop up on the onscreen keyboard in tiny, virtually impossibe-to-read small text, by default above the letter it thinks you may have meant to tap in the current word. (You can turn off this unreadable suggested text or have it appear above the spacebar instead.) And the ability to swipe these suggestions into your text is no less work than tapping the suggestion; the much-ballyhooed swipe-up suggestions seems like a gimmick created merely for demos.
The main suggestion appears in a large blue box that is easy to read -- but it's smack on top of the spacebar, hiding that character and preventing you from ending the current word if you don't want to accept the suggestion. (I had to tap a punctuation character, then backspace to delete it, and then type the spacebar.)
The onscreen keyboard is not smart enough to provide special symbols such as @ and _ when entering URLs and email addresses. (Sometimes, @ was available.) And there is no .com button.
Finally, the method to apply formatting to text in emails is unintuitive. Every time I tapped the Format button at the bottom of the screen, I got the onscreen keyboard instead. What you have to do is hide the keyboard (by tapping and holding the spacebar), tap the More button, tap Format in the contextual menu that appears, then tap the Format icon at the bottom of the screen to display the formatting bar. Crazy! After that, the Format button is active at the bottom of the screen, when the keyboard is hidden, so you don't have to re-enable it in the contextual menu. This seems to be a Hub flaw; in Documents to Go, I could just use the contextual menu for selected text to enable formatting.