There are several options to consider when selecting a Digital Voice Recorder. To keep things easy we have created a "Shop by Industry" section that makes the process of selecting the right recorder for your needs simple.  The 'Shop by Industry' section highlights the most popular recorders and features for each industry.     
For basic discussion we have completed a brief article outlining the general features to consider when selecting a digital recorder.
Note: The digital voice recorder market consist of two categories "Professional" and "Consumer". The "Professional" market includes Mobile Professionals, Healthcare Providers, Insurance Agents, Law Enforcement Professionals, Legal Professionals, etc.  The "Consumer" market consists of home users and/or individuals with minimal needs.

Record Time / Record Quality
The first thing to consider when selecting a Digital Voice Recorder is the amount of record time needed. Each Digital Voice Recorder has a maximum record time which is based on the memory (storage capacity) and the Record Mode. The Record Mode is very important as it determines the quality of the recording. 
For Example the Sony BM1 and a 16MB card would allow for the following Record Modes and Record Times:
LP Mode: 5 hrs 45 min  (lowest quality of recording)   Long Play Mode
SP Mode: 2 hrs 10 min  (better quality of recording)   Short Play Mode
ST Mode: 40 min  (best quality of recording)    Stereo Mode  
If you are recording in "High Quality or Stereo" Mode for example the recording will be clearer (better quality) but take up more space, thus the record time in this mode will be less than "Long Play" mode which will produce a less quality recording but will take up less space.   
This is important because many times the "Maximum" record time promoted in marketing is referring to "LP Mode" which may not render the quality recording desired.

Built-in Memory / Removable Memory
The next factor to consider when purchasing a digital recorder is the type of storage, built-in or removable.  Built in memory limits storage (record time) to a fixed amount while removable media allows you to remove and replace the card with a larger card (generally up to 512 MB) for greater storage.  
Removable media offers greater flexibility in other areas as well. For example if you need to have a file transcribed or downloaded to the pc you simply turn in the media card not the recorder so you can continue recording memos without interruption.  

Recorder Features
This is a more complex topic since there are numerous features and desired features tend to vary depending on industry. Thus we will only cover general areas.  
One of the first features to consider is basic operation of the digital recorder which can be done one of two ways through "Push Buttons" or a "Thumb Slide Switch" to Play, Record, FF, Rewind, or Cue. This is basically a preference issue but individuals that use their recorders frequently will find the "Slide Switch" is much easier and faster to use. 
Other feature examples include ability to lock a file to prevent overwriting, or the ability to insert a recording into an existing recording, create multiple recordings within a folder, and/or assign an Author ID and file type to a recording. Depending on your industry other accessories may be required such as a transcription kit which includes foot pedal and transcription software.  
And of coarse there is the over all size of the recorder. They are handheld recorders thus they easily fit in the palm of your hand, but some are more compact.  Again this is a preference issue; however we have found some recorders can be difficult to operate because the buttons are too small, close together and hard to manage.   
There are several other options that might be of interest. Certainly the software that comes with the recorder is important since it controls what you are able to do with the recorded files once they are downloaded to the computer. Features vary but almost all "player/editor" programs (software that is packaged with most recorders) allow you to play, edit, and convert files to other formats if needed.  
Finally, the great news is there are many different models and a wide variety of features making it relatively easy to find a digital voice recorder that will meet all your needs.