HDMI: If you intend to use your tablet as a presentation tool, and want more folks than those sitting next to you at the table to see what you're showing, you'll need video output. HDMI is by far the most common type -- and even the iPad supports it via a $40 adapter.
Camera: You'll definitely want a forward-facing camera if you intend to make video calls or do videoconferencing. The lack of this feature was a major drawback of the first-generation iPad and some early Android tablets, but forward-facing cameras are now almost universal, though resolution varies. If you want to impress your clients, look for at least 1.3 megapixels that will do 720p video, although you'll need a fast connection to use the higher resolution.
Look for the same resolution in the rear-facing camera, which is handy for snapping photos and for capturing documents and business cards using a scanning or OCR app.
Many tablets lack removable storage, so capacity can be an issue. Applications, e-mail, word processing documents, spreadsheets, and presentations don't require much space. However, if you deal with lots of video, you might want to purchase a high-capacity tablet (the maximum today is 64GB, on a handful of models), or at least make sure you can expand capacity via a micro-SDHC card slot. Many Android and all Windows tablets allow you to transfer and access data directly from external storage devices such as a thumb drive or a USB hard drive.
TheiPad, however, supports transferring large amounts of data directly from a PC using iTunes or from online storage services such as iCloud, Mozy, or Dropbox. However, downloading from an online storage account can be frustratingly slow on the road. Wi-Fi hard drives, such as Seagate's GoFlex Satellite Mobile Wireless Storage, are a faster option (in most cases) for the iPad.
A more critical issue is that, depending on the app and service you're using, you may not be able to do much more than view your document inside that app on your iPad. For example, if you download a Word doc, you may only be able to view it in the app, not open it in another app to edit it.
Security and data encryption are major issues for many business users, and for some companies, the lack of security will eliminate many tablets from consideration.
Windows 7 has Bitlocker (Ultimate and Enterprise versions only) and numerous third-party apps that implement security; Android 3.0 ("Honeycomb") and later offers system encryption; and the iPad has encrypted everything stored on it since day one. Android 2.x does not encrypt and is still offered on many tablets -- avoid those if you have data you need to protect.
Android, iOS, and Windows 7 all provide a means of establishing a VPN connection, so accessing your company network is easy as well. How many resources you can access is the issue. With Windows 7 and Android you can browse storage, but with the iPad you're limited to basic company communications, such as email and bulletin boards.
Lenovo has gone the extra yard with security considerations for its IdeaPad Tablet. Not only does it offer Cisco VPN and Good Technologies secure e-mail, it also lets you encrypt data on removable storage. It also ships with Citrix Receiver, so users can work with a remote Windows desktop under the control of IT. Your techs will love those features.
A tablet can meet at least some of your business needs, but it's most likely going to entail some compromise in the way you work. If you have software that requires Windows, then you need a Windows tablet -- no way around it. If you don't have that constraint, then the iPad and Android tablets are more finger-friendly options.