I found this bag of Starbucks coffee beans that expired a year ago. What happens when they expire? Are they less potent? Do they go bad?

The quality of the coffee goes down as time goes on. The roasted bean degasses after being roasted. You can see this effect when it is stored in a non-valved bag. The bag will tend to blow up in size due to the degassing of the coffee bean. The expiration date is just a date the roaster has deemed that their beans will still be good enough to drink and still have that aroma and flavor one would expect. However, the freshness of the roast is also determined on how you have stored your beans. It is recommended that you store your roasted beans in the freezer to slow down the natural effects that goes on. If this cannot be done, then you need to store it in a cool, dry location. But, you must keep in mind that if you want that great cup coffee, then you need to drink it as close to the roast date as possible. As far as finding the year old coffee, it depends on how it was stored and what you are willing to drink. I would recommend making a small pot and tasting a cup. Not the whole cup mind you, but just enough where you can determine if you wouldn't mind drinking the whole cup or not. If it taste good to you and there is no rancid taste, then it is probably fine for you. For me, I would throw it away as I am spoiled with drinking Kona coffee.

The quality of the coffee goes down as time goes on. The roasted bean degasses after being roasted. You can see this effect when it is stored in a non-valved bag. The bag will tend to blow up in size due to the degassing of the coffee bean. The expiration date is just a date the roaster has deemed that their beans will still be good enough to drink and still have that aroma and flavor one would expect. However, the freshness of the roast is also determined on how you have stored your beans. It is recommended that you store your roasted beans in the freezer to slow down the natural effects that goes on. If this cannot be done, then you need to store it in a cool, dry location. But, you must keep in mind that if you want that great cup coffee, then you need to drink it as close to the roast date as possible. As far as finding the year old coffee, it depends on how it was stored and what you are willing to drink. I would recommend making a small pot and tasting a cup. Not the whole cup mind you, but just enough where you can determine if you wouldn't mind drinking the whole cup or not. If it taste good to you and there is no rancid taste, then it is probably fine for you. For me, I would throw it away as I am spoiled with drinking Kona coffee.

Coffee is fine beyond the due date, however it is not going to taste great. The fresher the coffee, the better it is. However coffee doesn't go stale like say bread where it is unedible instead just the taste degrades.

Yes, I believe that taste won't be as great as fresh beans, but there is no problem using expired beans.

In reality really fresh coffee maintains ALL of its roasted qualities for only 10-14 days. The NZ roasters association recommends coffee should be consumed within 10 days of roasting.

OK, at the end of the spectrum I see coffee being sold in gourmet stores with 4 years shelf life!! So, it really depends how serious you are about your coffee. I personally think 1 month is still fine for freshness.