First and foremost, REI is a consumer co-operative. This means that every member of REI is actually an owner of the company. This is really the major difference between REI and other competitors. At REI, you know that you wil be approached by customer service representatives who are knowledgeable and friendly, and every representative uses the products, so they can speak truthfully about a product. REI's mission statement:"We inspire, educate and outfit for a lifetime of outdoor adventure and stewardship."
REI is also huge on environmental stewardship. REI hosts stewardship projects and clinics all the time to help clean up the outdoors and make it a better place to be. According to REI.com, they donate millions each year to environmental projects and organizations.
REI also has a 100% satisfaction guarantee, meaning that no matter how long it has been, you may still return a product if it does not meet your standards (with proof of purchase, REI members do not need to keep receipts for their returns). REI also has the most extensive product testing out of any outdoor retailer, so every purchase from REI is a quality one.
REI is also one of the 100 best companies in the nation, and has been for 10 years in a row.