Microsoft Hotmail and Outlook allow the user to view both his inbox and specific emails at the same time. This is very useful if you don't want to view every email in your inbox, but just click on the important ones. On Gmail, you either have to click through every email or go back to the inbox each time, which takes too long.
Which brings up second issue - Speed. Gmail often takes too long to do things. Maybe its because of all the labs and stuff I have enabled, but Google is supposed to be faster than this.
There are other basic abilities Gmail should add, such as better multi-send features, better formatting options and more. There are also random features that could be useful, such as "Deal-with-this-email-later" option, but that could be for another post.