I know it's personal taste as much as anything, but one of the biggest turn offs to me for a spiced beer is when the spices are too overwhelming.
Keep in mind that the potency of the spices is directly related to the OG of the wort (much like AA isomerization). So a big 1.090 Old Ale with some spices will need more than a spiced American Amber.
My favorite christmas ale recipe is based around an English Old Ale (dark, malty, a bit of coffee and chocolate on the back of the palate) but I toss 2oz of medium toast oak cubes into some high quality bourbon for a week with 1/2tsp of cinnamon, a bit of powdered ginger and a small dash of blade mace (I've found nutmeg to be a bit too astringent for beer in general). Let that sit for a week and then add to your secondary, sit for another week and then keg or bottle. It's fantastic.
http://www.brewersfriend.com/homebrew/recipe/view/28105/christmas-beeracle