What everyone else said:
- pilled
- stained
- stretched out
- no longer fits (and probably won't ever fit again! like if it shrinks horribly... but then I first try to pawn it off on my DD's!)
- ripped
- yellowed
- faded
- I splash bleach on them or wash the white ones with reds and they go pink, or with jeans and they go blue
I find that black and white knit tops wear out fastest. Black fades very easily, and white - no matter what it looks like when I pack it away at the end of the summer - ends up with yellowed pit stains, or just look grayed out. I have colored knits that I've had for several years. They get less wear than the black and white, which tend to need more frequent replacement - more like once a year.
Cheaper knits tend to pill more easily, but all knits pill eventually. Except for two BR long sleeve tops I've had for what seems like eons... they have a lot of spandex in them, so maybe that's the secret?.
If I have a knit top with interesting details that fits really well, I hand-wash instead of machine wash, to minimize pilling/shrinking/etc.
If something gets pilled or pit-stained, I have been known to wear a cardi or blazer over it and eek out some more wear. This works if you absolutely know you will NOT be taking the extra layer off!
Finally, when it gets too pilled/stained/etc, I turn it into PJ wear. This is my favorite tactic with cotton tanks and camis in particular. I will wear camis as PJ tops for a looonnnng time - typically until the dryer someday mangles the straps, chews them up, and spits them out....