When can't you fold pre-flop?

[mshalen]

I think you need to consider both pot odds and what will your next move be if you hit part of the flop.
If you are in the SB with 45s then the decision after the flop is easy but what if you have A3o and the flop comes AQT? You have top pair crap for a kicker with possible big hands and draws out there (especially with enough limpers to give good pot odds). Or what if the flop comes K93 rainbow, now you have botom pair top kicker. You need to ask yourself what kind of flop you want as well as what the pot odds are.