It's not that the iPad cannot do everything - it's that the process is cumbersome for some things. The best way to describe it is: The iPad forces me to change the way I do business to fit it. The Surface tablet (Pro, and to some degree RT) lets me do business as I'm already used to doing.
An example: Whether a popular notion or not, Outlook is the defacto e-mail program for most business professionals. If I receive an e-mail with file attachments (2 PDF files and a Word document), I want to save those files to a folder structure (like My Documents) to keep them organized. Then I want to open the Word document and work on it. However, it's filled with red-line and track changes from a legal team. Works great in Word 2013. Later I compose a new e-mail and want to attach the revised Word doc and 1 of the PDF files. I simply hit the paperclip and browse to the files - attach them as I normally do on my work PC. Another team sends me a link to a large file transfer (some due diligence on a project), which downloads through the web browser as a big ZIP file. I then browse to the ZIP file and extract the pieces I need - again organizing them in my folder structure. I can the attach those files as needed to an e-mail.
The iPad can do everything I mentioned above, but forces you to think and do differently. There is no attach option in an e-mail. You have to open an app first, that contains the file. In this case, Dropbox perhaps. Then find the file and send it to a new e-mail. It then starts a new e-mail with the file attached. But how do you attach a 2nd file, or 3rd file? Quickoffice Pro HD is a great program for editing Excel and Word files - but it's missing advanced features such as red-lining and tracking changes through versioning. The formatting is many times off as well - with paragraph structures and margins changing. And there are apps to deal with ZIP files, but you must first save the ZIP file out to an app (again, like Dropbox), or send the ZIP to the unzipping app, which will then let you pulls files out into another app (like Dropbox).
I understand why Apple wants to isolate people from the core file system, but in the business world we live and die with files organized the way we want them - with access the way we want it. The iPad could significantly improve if there was a file manager, and apps allows you to browse for files through the system. I've seen jailbroken iPads with some of that functionality.
Take for example the Android platform. Android tablets have a file system, and apps to explore it. Their software allows you to access those files (such as e-mail attachments). It's a bit closer to what I need for a typical work day.