Both in desktop and web-mail, drag and drop is nice but selecting multiple files in the attachment dialog is a must for most users. daily tasks are severely taxed by this.
Having lots of addresses and no easy way to sort them in the order you want is also bad for productivity. User ask for the possibility to sort by first/last-name or by company. Have address as list or card. ovarall choices.
And stuff they find in outlook like favorites and search folders are also asked for