1. My food is cold.  Take it back immediately.  Stick it in the microwave until just prior to plate melting, re-serve.  Offer to buy them water.  Yell at the chef.

2. My server has too many tables (usually from someone who spent a month working at Denny's and wants to make sure that their server doesn't get in trouble but that it is management's fault, just like it was when they couldn't handle their 10 table section that one breakfast shift from hell 20 years ago).  Give them your manager keys.  Tell them if they can do the job any better, live it up!

3. There's not enough liquor in my drink.  (Usually slurred.)  Return drink to bar, fill glass with ice.  Pour their drink into new glass.  Dip straw in Bacardi 151.  Re-serve.

4. Your women's room is out of toilet paper. (I actually appreciate this one!)  Ask the busser to re-stock.  Get a system, fool...you want people walking around your restaurant with un-wiped rear-ends?

5. We waited 2 hours to sit down and now we've waited an hour for our food. (A. It's was never two hours. B. It's always 20 minutes.)  Bring them some bread.  Yell at the chef.  Buy them dessert.

6. The girl I called said you didn't take reservations.   Say, "Well, perhaps she should have been more clear. What she meant to say was, 'No, dumbass, the 7pm Saturday night reservations are all filled right now, maybe next time you can call a little earlier than 6:30 pm the night of!"

7. You should have (fill in the blank) on your menu.  Give them your business card.  Say, "Give me a call when you open your restaurant so I can come try some.

8. This (fill in the blank) is too expensive.  Agree.  Tell them it is a shame we live in such a money-grubbing capitalist society.  Go on and on about how Marxist philosophy was so right-on, and if only the idiot Soviets hadn't been so corrupt, we'd all be drinkin' that free bubble-up and eatin' that rainbow stew...

9. Your portions are too small.  All kinds of possibilities here.  Better than the obvious "No, your mouth is too big", begin to cry, go get the chef and have him make a scene about how he spent the entire morning prepping each item so very carefully so they could enjoy their dinner, only now to be so very disappointed that they are not happy.  Offer them some free mashed potatoes to fill them up. 

10. Your portions are too big.  Bring out a to go box.  Say, "That's why we supply these, no charge!"  Yell at the chef.

Honorable Mention

My food is too spicy.  "Yes, that's why it says 'spicy' on the menu." 
It's too cold in here.  Turn the thermostat to 100.
It's too loud in here.  "What?"
These shrimp aren't "jumbo".  "We serve a non-mutually-exclusive menu."
I make better at home.  "Go there!"
Why can't I use this coupon with this coupon?  "Because then I would owe you money for your meal!"
This isn't well-done enough.  Take back, put on grill, stack weights on top, re-serve.
This isn't rare enough.  Immediately remove.  Go to freezer for new burger patty.  Wave over grill.  Serve.
This tastes too fishy!  Tell them you gave Mrs. Paul the night off and were forced to serve real fish.
Our waitress is rude.  "You would be too if you had me as a manager!"
My ice water is too cold.  (Yes, it's true, someone actually said this.)