The Real Estate Journal has a good short article on renovations to avoid.  I have used the list and added my own views to the list.  Avoid these if you plan on selling the home soon.

1. A Swimming Pool -  In the Phoenix Metro Area Pools are good but only about 55% of the buyers want them.  It's usually a good deal to buy a home with an existing pool, but you will never recoup your cost if you put one in yourself.  Just make sure it's something you are doing for your own enjoyment and not for resale.  It's different for luxury homes.  Often you will gain more in value on luxury homes, as more buyers want a pool in that case.
2. Home Additions - If you plan on an addition you can gain a lot of resale value and recoup your costs with a good profit.  Just make sure that the architecture of the addition matches and looks good with the existing structure.  If it doesn't match you will hurt your resale value.  Also look at the neighborhood.  Don't overbuild your house.  If it is much bigger than the neighboring homes you will not gain as much value.
3. Trendy Finishes - Remember Green marble floors!  This will never be good idea.  Do it for yourself but be aware that it will cost you a renovation before you sell or you will get much less for your home than if you had more mainstream finishes.
4. A Jacuzzi - The same as a pool but even worse.  This is a complete waste of money for resale.  Do it for yourself only.