At the end of the day, the owner of this domain name let it expire and didn’t renew it. So while there may be many legit reasons as to why they might’ve done that, one of the not-very-legit reasons could be that the domain is “dead” in some way or another. In other words, it’s no longer useful for the original owner because of a penalty or otherwise. If so, it's unlikely it'll be of any use for you as well.
Listen to expert domainers on domainsherpa.com. Read anything by Michael Berkens, Frank Schilling, Eliot Silver, Morgan Linton, and Ron Jackson. Watch YouTube videos on domain investing. Read blogs like The Industry News Magazine at DNJournal.com. Search #domains on Twitter. Go to industry conferences. Understand the Google Keyword Planner tool inside and out.
Everybody going to start using old domains should also know how to get them, I'm going to post soon article about it on seomoz as there are several ways starting from online rankings and lists up to dedicated seo tools that you can run on your computer and get precise and actual data on demand for any domains lists. Anybody intersting in such tools please visit our website: http://en.exdomain.eu/
Dictionary Word/Pronounceable/Memorable/Brandable: A dictionary word domain name is very valuable. Of course, the more popular and widely used the word is, the better. For example, “marketing.com”, “cars.com” and “hotels.com” are 7-8 figure domain names. Even if a domain name isn’t a dictionary word, being pronounceable and/or memorable and/or brandable are all big pluses that add to its value.
The domain name industry is quite similar to the real estate industry in a lot of aspects. There are end users, brokers, consultants and domain flippers or “domainers”. Domain flipping works similarly to buying a house, renovating it (or even sometimes just sitting on it) and then selling it again at a higher price point. The gist of it is: you’re purchasing a domain name and betting it’s worth (or will be worth) more than you paid for it. If you’re right, you get a nice paycheck and move on. Those who make a living out of this just rinse, repeat, and scale.