I'd suggest two additions - First is tell the prospect the reason they should call the rep back today. Second is that sales reps need to script these voice mails prior to the call. They should have their 90-words-or-less compelling voice mail scripted out so it sounds great. Practice practice, especially when leaving VMs after cold calling will ensure the messages sound natural and not scripted.

and I'll add my #1 rule for leaving a voicemail message.
Never give them a reason NOT to call you back.

Whenever someone calling me cold asks for a call back "today" it's pretty much guaranteed that I won't. Asking for a call "today" implies that you are trying to establish control, which is a turn off. You establish control by conveying "success" and "importance" with your voice and words which then compel people to call you back.

@Jim. Jim, I agree with your posting regarding using the word "today" in a voice mail. It is tricky BUT you have to take into consideration tone. If you emphasize the word TODAY like you are yelling at the person then absolutely it comes across as agressive. But if you de-emphasize the tone it can work as a subliminal suggestion and be quite effective. Different things work for different people and that is why sharing is so important! Thanks for your input...keep it coming.

TONE - the key to all sales calls! Most calls I get are "please get back to me TODAY" - no good. But, "Jim, if you could get back to me today I would really appreciate it" - that can go far.
Meanwhile, "tone" is most important on any sales call - something I learned the hard way from my wife and mother.

A sales prospecting campaign that I read about actually championed the use of Voice Mail. If you are interested in something truly out-of-the-box, and successful, you can read about it on my CustomerThink Blog, "How Humor and Eavesdropping Combine to Win Sales." Here's the link:
http://www.customerthink.com/blog/how_humor_evesdropping_combine_win_sales

I know this may seem obvious, but speak clearly (and slowly) and leave your phone number!!! So many people will call, say their name at the beginning so quickly, that I have to listen to my message ten times to figure out who they are. Then they leave their phone number, very quickly. People don't understand that when you leave a phone number on a voicemail, the person listening will be writing it down. I find it helpful when a candidate closes their voicemail by saying 'my phone number is .... again, this is John Smith, and you can reach me at ....."

Have a laid back and clear voice with a slow tempo. Sound confident and relaxed. Introduce with your name, but at the very end, leave your name and phone number so they had time to grab their pen.

What are the reasonings behind the don'ts?

Refer to your company as the industry leader - reason - your message should not be about you and using words with no value take up time that could be better used to arouse curiousity. 
 
 
 
Spew your company history or name drop more than 2 relevant customers - reason - same as above...it is not about you...use this time to show the prospect you know something about them. 
 
 
 
Reference the fact that they recently attended a trade show or downloaded a white paper - reason - how boring...how does this convey you know anything about their company, job, challenges etc.? 
 
 
 
Ask for a commitment of their time before you have established credibility - reason - time is the most precious commodity...asking for it before you have established that you can add value in some way, shape or form is counter intuitive. 
 
 
 
Leave your email or web address in your voice mail message - reason - you want them to call you back so why give them a reason not to? 
 
 
 
Hope this helps Merry and thanks for asking!

Hi, I'm Mike and I'm a reformed anti-voicemail outbound dialer. 
 
I've even gone so far in the past to train people in my company with a slide that said "Voicemail is for Whimps". Then I saw the light...well, I listened to Trish and others and decided to experiment.  
 
I setup an A-B test with two of my reps. I'm now a believer.