I've grown quite fond of my Android based pad lately and use it for quite a number of things from web browsing to book reading. However, there are some limitations that will hopefully be removed in upcoming software versions.
Researching something usually involves opening a number of web pages, documents and PDF files, to switch between them quickly and to search in them. All of these things are not ideally implemented yet as the user interface is designed to only show one application at a time. Sure, Android is multitasking and holding down the home button for a second shows the list of applications one can switch to but it is too slow. When the mind is racing and making cross connections, you don't want to wait for a second.
Also, my web browser and pdf document reader only show one page at a time. In case of the web browser it is possible to open several pages at once. However, switching between requires swiping in a menu and then selecting the other page. A tab like on the PC is much more convenient for this. And the PDF reader, well, it can only open one file at a time. Not ideal at all for switching.
Obviously, most people will use a pad to perform one task linearly, but it has potential for much more. For the moment, however, I think this is the big limitation for me. Even if you add a keyboard and thus make a pad "almost" into a netbook, the ease of switching between applications and doing things simultaneously are nowhere near what you can do with a full operating system. Quite on purpose but it doesn't help me.
When spinning this thought further I think this is also the rift between pads and small mobile devices. On mobile phone type of devices, I definitely don't want tabs in apps or an always visible menu to switch applications quickly. Here, the screen is just too small. But on a tab, the screen is by far big enough to allow for all that. Having one operating system for both is challenging and requires at least a customized UI for the different device types.