Where can I find honest, independent assessments of online discount
stores such as Plasma House, Get Everything.com, Best Digital, and the
like? The prices of electronic equipment at these sites are so much
lower than high-end stores such as Harvey's that I've got to believe
there's a downside to ordering through them. Other than treading the fine
print, how do I find out what the downside might be?

One way would be to read message boards and opinion sites that
evaluate such stores, for example (but not only) www.epinions.com

It all comes down to if you trust them.

You should have to be reading all the 'fine' print on a website.  If
the E-tailer is trustworthy, everything they say and do in terms of
the presentation of information on their site will naturally lead you
to that conclusion.

Are they up front about:
1. Their shipping fees
2. How they protect your privacy
3. Returns policy
4. Damage policy
5. Product guarantee
6. Shipping times
7. Did they make it easy for you to buy and help you to make an
informed buying decision
8. Are they up front about who they are and where they're located (or
were they secretive in any way [i.e. no phone number, only P. O. box
address, etc.]

Ask yourself these basic questions and any others you can think of. 
All of these answers should be no more than one click away.  If you
can't find it, its a red flag and you should move on.

You can easily find if other people have complaints or have been
ripped off by a company by doing a simple search.  Search for phrases
like "company abc complaints" or "company abc ripoff," etc.

Many times online companies can offer lower prices than their
brick-and-mortar counterparts.  They do not have the overhead costs of
running a physical store, paying salespeople, etc. and have the
ability to pass these savings back onto their customers.

Of course, let your common sense be your guide.  If the price is 'too
good' to be true, it probably is.