Spears win the "most common weapon with the most special rules in gaming" prize. They can impale you and get stuck (Runequest); have some kind of an initiative or reach advantage due to their length (just about everyone); can be set against a charge for an insane amount of damage, which is only fair because a charge itself does an insane amount of damage (AD&D); can be used from the second rank (OD&D). I'm probably missing a couple more.
Starting with the spear, there's definitely a difference between the long and short spear. The short spear doesn't have reach but can be thrown. The long spear does have reach but needs two hands; balance that and the lesser damage of the spear, against the possibility of a tight formation (3' frontage) and whatever reach gives you. Simple enough.
Impaling is part of critical hits, which my system isn't doing.
Charging is one of those things really more relevant to mounted combat. You would only get a substantial bonus on the attack when moving very fast and seated with the lance couched. As for setting spears against a charge - again, only really deadly against mounted charges, and with more a deterrent effect than anything else. In a skirmish, a horseman or other charging creature who could see you holding the spear could usually veer off in time. Still a useful rule to have.
Three rules for the two-handed long spear, 7' or longer:
Long: You strike first against one opponent with a shorter weapon on the first round he moves into melee with you, if you're facing him. Deep: You can attack from behind a friendly character, without the first strike bonus. Set: If you are wielding a long spear, and you are involved in a charge at greater than 12" movement rate, you strike first and the spear has +2 to hit and 1d6 extra damage. This is true whether you are charging or being charged. If two spears clash in a charge, both strike simultaneously; this is one place where the "back and forth of combat" model doesn't apply.
One other thing to keep in mind for any pole weapon. Although it can be used in a narrow frontage, to turn and still use it will require room equal to its length, either to the side or overhead. This makes it a risky choice in a dungeon setting.
Researcher in social science and appreciator of hexy wargames, role playing games, interactive fiction, board games, and CCGs. I've learned a lot doing design and rules editing work on the side for the Legend of the Five Rings CCG. But this blog is mostly about other things.