Zipher said:
Can I choose not to use a feat slot when it becomes avalible? instead, can I "hold on to it" until the next time I level?
Here's what I think.
A PC is a person who keeps learning stuff throughout her life. When she "gains a feat" by "levelling up", she is not suddenly enlightened into the knowledge of brewing potions or fighting with a new weapon, just like she doesn't suddenly learn new spells or gain improved abilities; levelling up is an abstraction done to make game bookkeping easier, but obviously your PC has actually learned or trained for the improvement during the previous months or years, and finally getting the stuff is the result.
Feats in particular are usually gained every 3 levels. This means that basically you have been learning the feat during your previous 3 levels. So for example if you got Scribe Scroll at level 3 and Brew Potions at level 6, you could imagine that your character has actually spent levels 3, 4 and 5 learning how to brew potions. That obviously is only a mental image, because perhaps she has been studying it for the last 20 year instead
That said, I don't think it's a real problem to allow someone to delay a feat. If a player asks the DM that it's either (1) because she currently has no idea about which feat to take

or (2) because she is making a trick to avoid some rule limitation.
In case (1) it should be the DM's responsibility to help the player. If it would interrupt the session, it's definitely better not to force the player choose the first feat on her mind! I'd just make a deal with her and help her choose the feat at the end of the session, or before next evening. It's kind of obvious that NOT taking the feat (or any other ability) at the first opportunity is a loss for the player, not an advantage.
However, there are very few circumstances when it's actually an advantage. Sometimes you cannot get a specific feat because you don't meet the prerequisites yet, such as Weapon Finesse at level 1 if you are a Rogue. In this case it would help to delay the feat until level 2, because if you take a different feat, the next time you can get WF is level 3. So some players ask for feat delay just because of this.
A strict DM would probably forbid this, and the authors definitely support this view, as they voluntarily designed feats requisite to make them unavailable to certain classes before a certain level.
But you know what? I wouldn't even mind to allow such a thing

I don't think that voluntarily delaying NOW the acquisition of a feature, to gain it LATER a little earlier than expected makes any problem to the game. Actually it's quite a fair trade-off. There could be specific cases to forbid, however.