First, it should be inexpensive, within the range of $100 to $150.
Second, you should be able to use it without assistance and, ideally, be able to just slip it on.
Third, it should use dry electrodes. You shouldn't have to apply paste or saline solutions. Currently, the dry electrodes used in the more inexpensive devices cannot be used where there is hair on the scalp. It would be nice if some hairy sites could also be used, but that may be too much to ask at this point. (See examples of wearable devices in development http://www.imec.be/ScientificReport/SR2009/HTML/1213373.html.) The picture below shows one I like because it allows for hairy sites.
Fifth, the software included should provide access to the raw EEG and use a standard such as EDF so that MatLab and EEGlab can be used to analyze the data.
Six, it should have drivers for third party software such as BioExplorer and BioEra.
I am sure I will think of more.
Please feel free to comment or, if you wish, direct questions and comments to me directly at drampsych at gmail.com.