Laptops and cellphones are full of protective devices designed to keep you from ever damaging the batteries them selves. Even then, it still happens once in awhile where they'll go off. Just fairly recently I believe it was sony(?) who recalled all kinds of laptops due to problems with the lithium cells igniting.

In our case, not only do we not use protective circuits, but we also use the batteries way out of their intended specification range. Most cells simply aren't designed for the 20C+ discharge rates we subject them to. This is why in lab results, lithium batteries can produce up to 500+ cycles with less than 20% capacity loss, whereas with the way we use them, you're lucky to see over 100 cycles before the pack starts to die on you.

Over-discharge / over-charge (and more specifically, the heat that is generated from this) is what is most damaging to a lithium battery in use aside from physically damaging it or shorting it. Heat is your packs number one enemy. If the pack is getting to hot (greater than 120F) you're pushing it too hard. 140F is about max. Once you start to exceed 140 cell life goes down considerably. I think they become a fire hazard at around 180F (Don't quote me on that, though!).

Cell life is also affected quite considerably if voltage is allowed to dip below 2.75 volts / cell, or higher than 4.2 volts / cell. With the latter being more likely to cause the pack to ignite (during a charge).