In the interest of transparency, here's how I play wishes.
The factors that affect how a wish will be granted include:
- How selfish is the wish? Is it motivated by a desire to harm, or to help?
- How ambitious or grandiose is the wish? How much would reality have to change to grant the wish?
- Does the wish duplicate or improve on an existing spell?
- Is the grantor of the wish friendly, neutral, or malevolent?
The less ambitious and less selfish the wish, the more likely it is to be granted in the spirit in which it was made. Wish for a turkey sandwich and you'll get a really good turkey sandwich. Wish for everyone in the party to be restored to full health, that'll go off without a hitch. Try to wish for everything and the kitchen sink and you're likely to end up wishing you'd wished for a turkey sandwich.
Wishes in my campaign will often follow the path of least change. Thus, the classic "I wish everyone else were dead" wish is likely to send the wisher far into the future (and out of the campaign) because sending one person into the future is less of a change from the world's perspective than killing everyone in the world. It's also motivated by a desire to harm, of course.
Wishes that duplicate or improve on an existing spell will usually work as planned.
There are some things even a wish can't do (for example: disarm most Gygaxian traps). There is no set rule on what wishes can't do, but assume there's a certain amount of plot armor. Wishes to go straight to the adventure objective won't work because there'd be no adventure. Such wishes usually fall into the category of "overly grandiose," too.
Finally, conjunctions
may require multiple wishes. "I wish for a million gold pieces and eternal life and a hot girlfriend who's into the weird stuff" is three wishes, not one. The method of wish granting will determine what happens to wishes like these. A ring or similar device might grant only the first clause or might not grant anything at all. Something with its own agency, like a djinn or efreet, might choose which part of the wish to grant or might count such a wish as multiple wishes. Combine multiple goals into a single wish at your own peril.
To sum up: the simpler and less selfish the wish, the more likely it is to work.