Based on my limited experience:
Meal vouchers have a price limit and can only be used at certain places including most airport restaurants.
Free trip vouchers either have blackout dates or cannot be used to make reservations MORE than a few days in advance. They are almost never good for a trip to Hawaii.
Often the compensation voucher is for a dollar amount rather than a free trip in which case it can be used towards any published fare. Usually it cannot be used for an on line purchase.
Volunteers getting off the last flight of the day are almost always given hotel vouchers along with dollar amount vouchers or trip vouchers. Involuntary bumped passengers who insist on cash back usually do not get hotel or meal vouchers, but will get the same thing volunteers get if they prefer that.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm
Example 1: United Airlines, Dulles (DC) to San. Fran. Got round trip voucher with max. 24 hour in advance of first flight reservation, dollar limited meal voucher, hotel voucher.
Example 2: Delta Express, Orlando to Boston. Got dollar amount trip voucher, hotel voucher, no meal voucher.
Example 3: Delta Express (I wasn't chosen) Dollar amount trip voucher larger than mine in #2 above, no hotel voucher.
New rule for some airlines, if approved hotels are sold out, you get a dollar limited hotel voucher. Many other hotels have a "distressed traveler" rate, you should ask for that when making reservations at the phone bank at the carousel.