Question #19 on Jeff's list is "Any legendary lost treasure I could be looking for?" This is largely a setting-specific question; each campaign can develop its own artefacts that come from the history of the setting. Let's deal more with the types of "legendary lost treasure" that could be lying around. Also, think about fragmenting some of your treasure, leaving pieces of one treasure in all sorts of different locations; only when all the pieces are brought together does the treasure work. This is, of course, a classic trope for high-fantasy, save-the-world type MacGuffins, but why not make that the case for your plot-neutral artefact too? Scattering an artefact in multiple pieces makes characters (and therefore players) have to work that much harder and have that many more adventures whether assembling the artefact saves the world or just lets the character fly or breath fire or swim in magma or whatever. It's the journey, not the destination, so why not make the journey three or four times as long?
Answer 1: MacGuffins. These types of treasure are basically flavored "plot coupons." They don't do anything for the PCs, but if the PCs can get their hands on the treasure, or get the treasure to some place, or do something with or to the treasure (possibly with a deadline), the events of the game will change in some way. Some Dark Lord will be destroyed, some guy will win the crown, something will be saved, something will be destroyed, etc.
Answer 2: Mechanically helpful artefacts. If this is "legendary lost treasure," make sure this is good if it's only going to give mechanical advantages. So far as the genre goes, swords seem to be the common item that falls into this and the next category, and even some in the first answer; that's probably because swords hold a special place in our romantic thinking because only nobles and the very rich had swords back in the day. Swords used to be so expensive that barely anyone owned one, so of course great legendary items were swords.
Answer 3: Roleplaying enhancing artefacts. These grant advantages to the wielder in ways better described with words than with plusses and minuses. Think Telecanter.
Answer 4: Riches. Maybe legendary treasure isn't an item in itself, but a hoard of ancient gold bullion or coin buried away someplace, or maybe just artefacts that are worth a lot of dough on either the general market or to a particular person. Either way, the main point of this treasure is increasing the amount of gold the PCs need to have taxed out of them.
Answer 5: Something relevant and helpful to a class other than a fighter. In a game set in Vance's Dying Earth, for example, the ancient pioneering magician Phandaal's lost spellbook would be a great legendary treasure (and, in fact, a lesser spell book and magic were the basis of a quest in one of Vance's stories). Just don't hand something like that to low-level characters; make their players work for it, seeing as it's the comprehensive collection of every single spell known by humans in the setting.
Answer 6: A surprise! The PCs think that the treasure is one of the types of treasure I've already listed, but in fact it's something else of similar magnitude. Usually this will be something bad, like unleashing an army of zombies/robots/recently-de-petrified-terrasques or a MacGuffin for the PC's enemies, or just a really bad cursed item, but it could also be something good, like, now what remains of the ancient robot army is under the PCs' command, or just a really good magic weapon they weren't expecting, or some-such like that.
Is there anything else that you can think of that legendary lost treasure can affect the PCs?