FAQ: How strictly should I interpret the wording of a Wish?
Part of the problem with figuring out how to resolve this issue is that the thing which "grants" a wish (like a ring or spell) is often not itself intelligent. This makes it difficult to figure out how literally to take what the PC asks for. I think the solution is to have some sort of intelligent entity grant the wish.
One approach is to have the wish granted by a powerful NPC who is grateful for something the PCs have done. By doing this, you can figure out how to handle their wishes based on the personality and alignment of the creature making the offer. For example, if the creature making the offer is good and is sincere in his offer, then he would overlook minor errors in the way the wish was phrased and might even seek clarification from the PC before granting the wish. If the creature had a mischieous bent, they might grant the wish, with a harmless or amusing twist. You could also have a creature who was not sincere and is looking for a mistake in the wording of the wish that he could twist to his advantage. I wouldn't do this without giving the player plenty of clues that the offer might not be what it seems, though.
Handling wishes this way also allows you to limit the power of the wish in reasonable ways, since not even gods are omnipotent in D&D. So if a player wished that "all my enemies be slain", the creature granting the wish might reply, "Even I can not defeat the combined hordes of evil you face, but I can provide you with this (insert powerful magic item here), which may assist you in your battle."
And of course, NPCs might not be willing to grant wishes which were contrary to their alignment or interests or which they consider to be too greedy.
You could also use this approach with magic items like rings by having the ring be intelligent. In that case the being in the ring would decide how to handle the wish. The players could also find a ring that they believe is an ordinary Ring of Wishes, but when they try to use it, they find out that what it really does is summon some powerful creature to grant the wish. This might be a diety of some sort or just a powerful mage, who could cast any spell up to a certain level.
You could also introduce other effects to make things more interesting. For example, the ring could perform a time stop as part of the activation. This would allow the PC to discuss his wish with the granter before it takes effect. This would be valuable if they used a wish during combat, for example. So as soon as the player utterred the words "I wish...", you could stop them and take them to a different room where they and the NPC could discuss what the NPC might do to help. Then you can return to the rest of the party and announce what happens. That could provide an opportunity for some interesting role playing as the other PCs try to figure out what just happened.
Unless you are running a very high level campaign and have a PC wizard who can legitimately cast the Wish spell, wishes are something the DM inserts into the campaign at his or her discretion, usually by putting a magic item such as a Ring of Wishes as treasure. And by having an intelligent being decide how to handle the wish, I think it makes the DM's decision seem less arbitrary.