This actually happened a while ago, but I just remembered it.
A player is building his sorcerer for my Pathfinder game and finds a new spell on the SRD. I check it out. It's a pretty neat spell. Except it's also not.
I'm making this up, because I don't remember the actual spell, but in essence it goes something like this: you summon a magical weapon that remains there for three rounds. Each round while it is present you can make an attack with it. The first attack does 3d8 damage, the second 2d8 and the third 1d8. And I'm like: Why? The first 3d8 is not bad, I guess. The next round, you're probably eyeing that 2d8 and considering just casting another spell instead. When it goes down to 1d8 you're probably sighing and spending your action in a better way.
Why would you make something worse the more a PC uses it. Not just "wear out until it's spent" in a resource-management sort of way, which is cool, but actually get worse. You're PENALIZING the player for continuing to use it. Which, to me, says: "don't use this spell, silly."
2d8 for all three rounds would be fine, with the same average damage. 6d8 divided amongst any number of attacks (max 6) until it runs out would be pretty interesting. Maybe you can try to blow it all in one hit on the first round, maybe you keep attacking for six rounds because you don't have a better weapon. It's also a tactical consideration.
Best of all, why not make it 1d8->2d8->3d8. Boom, BOOM, BOOM! is more fun than BOOM, boom, pfft. It encourages the player to use it for longer. It tempts you. Come on, attack again, it will do more damage, it whispers.
When you make a spell, feat, class, power, whatever, remember that it will be used by real people playing a game and consider what effect it has on the people at the table, not just in the game.