No addon can find out what quests you've completed unless it is running when you turn them in. When you start a guide midway through you have to manually check off the quests you've done. There's simply no way around this.

major major ditto... this is good for both leveling a friend (or wife's) alt, as well as getting that dang faction rep when you are so bored that maxing out all faction rep seems a good way to pass the time.

also, and i doubt it does this, but i would LOVE an addon that lets you know what quests others in your party are on. sure, you might all have to be running the addon (an acceptable price), but that information would be so very valuable. instead of constantly asking "can you link them again" (and even THAT no longer works since yesterday)

@Tekkub: given a list of all avail quests for your faction/race/class/proffs/level (via something like wowhead) would it be possible to cross ref quest givers with npcs offering quests? if this test can be done then it may be possible to make an estimated guess at the completion of a quest based on if the npc still has it to offer. would require the player to get in range of npc i assume, but better than nothing.

"given a list of all avail quests..."
Well, first off you have to talk to the NPC to find out what quests they have. TG+TomTom+LightHeaded will map the NPC for you, so by the time you get to him you can see he doesn't have the quest, remember that you did it, and manually check it. If there were a way to know without interacting directly it could work, but there isn't a way and never will be.

Also, wel you have to load up a lot of data for this, and that data would need to be very accurate. I've seen a lot of bad or missing required levels on the DB sites... and there are a large number of quests with prereqs that aren't listed at all... good thing those sites let users post comments.

In short, it's a lot of data to load, you still have to talk to the NPCs... and really you only need to go thru a guide once when you first start. After you've done a level or so in TG it all flows quite nicely, unless you go off and do things before the guide tells you to.