It also starts the clock for your boss and your company, kicking off the transition plan of training, hiring, and otherwise covering your impending departure. The bad examples here are all too vague. With the last one, you're aligning yourself with company policy, which is good, but it still doesn't tell the reader how much notice that actually is. Be as specific as possible on the time you have between now and your last day.
Tempting You to Stay. It's a reaction that's both good and bad: Your boss values you so much that he or she offers you a raise or promotion to stay. But it can leave you feeling conflicted: You have another great job lined up__but now that there's more money on the table, do you really want to leave your current position?