One neat new feature is Thunderbird's Attachment Reminder, which looks at the message you're composing, and if it detects the word "attachment" but no actual attachment, prompts you to add one. But you lose Yahoo Mail and Hotmail's ability to store large attached files and photos on Web storage rather than filling up your recipient's inbox with many megabytes. And you get basic text formatting and a smiley dropdown, but there's no stationery like that in Windows Live Mail or Yahoo Mail.
Like pretty much every modern mail client, recipient e-mail addresses will autofill if you start typing one of your contact's names, and your drafts are automatically saved. A spell-check button assures you of your orthography, and it even works on the fly as it does in Outlook (you know, the squiggly red underlines).