Join Date: Jul 2010
Location: State of Confusion (SC)
As a former HR person, mtngrl7500 nailed it.
Keep it short.
If you want to ever be considered for another job there, mention that you enjoyed your job and the people you worked with.
It's not necessary to go beyond that. If they ask, you might give a reason for leaving...make sure it's reasonable and doesn't reflect badly on the company.
If you want to, you can certainly add it to the letter, but it isn't necessary.
What IS NECESSARY (esp. If you ever want to be considered for rehire...or want a reference from them) is as much notice as possible. 2 weeks is great.
If you are a professional, make it a month.
I'm not a complete idiot--there are parts missing!
What you have become is the price you paid to get what you used to want.