Absolutely has to have multiball, but I don't really like games that give multiball out easily, I feel it should be something you have to work hard to get.
Gotta love some great ramps, like WH20
Mechanical toys are a huge part of the interesting things going on under the glass for me
Like deep rules for my games although makes it hard when playing a game on site and not familiar with the game
Love fast flowing games with satisfying loop shots, when you nail a few in a row great feeling 
One thing that is not on there which I love is a great light show for jackpots etc. like HS2