It's an italian basketball team's logo, but there aren't a lot of gaming images for the spell!
First of all, consider its peers. Sixth level MU spells include such things (among others) as death, disintegrate, control weather, flesh to stone (and its reverse), and reincarnation. These are some pretty powerful spells in the BX universe. Magic-users don't see them until 11th level and they aren't even available to elves. So why is geas among them?
Well, first of all, in BX it has no set duration (unlike later, wimpier editions). A geas can theoretically last forever. Secondly, the geas "forces the victim to perform a given action or to avoid performing a given action." So the caster can not only make a target go do something (even serve him for life!), he can also require him to refrain from things like reporting crucial intelligence to his king, ever leaving his tower, etc. Remember, it is a single task or action. The wizard can't lay down too many conditions and side notes. GM discretion is warranted here. Of course, there is nothing preventing multiple geases.
Another fun twist is that if the geas is deemed impossible or "directly fatal" then it rebounds on the caster. So it's fine to tell the victim to go and bring you the head of the vampire lord, but you can't have them jump off a cliff.
It's worth noting that -like quest- only the reverse of the spell can remove the geas. Dispel magic or remove curse won't cut it.
It's interesting that the exact nature of the penalties inflicted upon a disobedient victim is up to the GM, not the caster. Personally, I would enjoy input from the player for ideas, but might modify it if I thought it was necessary.
The assumption in the spell description seems to lean more toward the geas being a task to be done, but that's not always the case. One of the examples in the spell's description is "to never reveal certain information." Say a character is geased (is that a verb?) to never reveal the location of the wizard's lair. A secret once told is lost. So either the penalty needs to prevent blurting it out or punish breaking of the geas harshly afterwards.
To continue the secret-keeping example, I would say an attempt to reveal the information might cause wracking pains that prevent communication and reduces the character's CON score by half. Further attempts to say or write or otherwise reveal the information would reduce their CON to zero and kill them. The target has already lost his saving throw, so the penalties are automatic (6th level, remember?). Even an ESP spell used against him would be punishable, IMO.
In the case of a task to be done, the penalties might follow the more gradual route. Of course, the other possibility would be to assign a deadline or some such. "Kill the dragon before the next full moon or ____!" is simple enough. You have X days or the geas' penalties kick in. Maybe you die, maybe you become a slave, or a pig, or whatever.
Finding a scroll or spell book with a copy of this would be quite the prize. Frankly, if I ran a game where the PC MUs had to find their spells, I would make this one a -heheh- quest of its own.