I noticed there doesn't seem to be any visible links to the old (but still useful) GTD threads, so, in the hope that it is of some use to people other than I, please find below a collection of GTD's I have found by scouring the forum:
GTD: What's wrong with adventure games? (http://www.adventuregamestudio.co.uk/forums/index.php?topic=7763.0)
GTD: Creating Atmosphere (http://www.adventuregamestudio.co.uk/forums/index.php?topic=11341.0)
GTD(?): Handheld Adventure Games (http://www.adventuregamestudio.co.uk/forums/index.php?topic=18486.0)
GTD - on Dialogue options (potential spoilers) (http://www.adventuregamestudio.co.uk/forums/index.php?topic=47112.0)
GTD: DLC (http://www.adventuregamestudio.co.uk/forums/index.php?topic=43757.0)
GTD: The Language Problem (http://www.adventuregamestudio.co.uk/forums/index.php?topic=28803.0)
GTD; On game worlds (http://www.adventuregamestudio.co.uk/forums/index.php?topic=25995.0)
GTD: Backgrounds and "mise en scene" (http://www.adventuregamestudio.co.uk/forums/index.php?topic=10957.0)
GTD: Media Analysis of Games (http://www.adventuregamestudio.co.uk/forums/index.php?topic=12697.0)
GTD The Importance of History (http://www.adventuregamestudio.co.uk/forums/index.php?topic=16171.0)
GTD: How are game aspects linked? (http://www.adventuregamestudio.co.uk/forums/index.php?topic=27088.0)
GTD: giving personality to characters (http://www.adventuregamestudio.co.uk/forums/index.php?topic=8248.0)
GTD: analysis of game verbs (http://www.adventuregamestudio.co.uk/forums/index.php?topic=17812.0)
GTD: Minority characters in adventure games (http://www.adventuregamestudio.co.uk/forums/index.php?topic=8033.0)
Unbranded GTD's
Voice acting or not voice acting? (http://www.adventuregamestudio.co.uk/forums/index.php?topic=15053.0)
What do you expect from a good GUI? (http://www.adventuregamestudio.co.uk/forums/index.php?topic=15845.0)
~ The future of multiplayer adventure games ~ (http://www.adventuregamestudio.co.uk/forums/index.php?topic=12421.0)
There is of course an old Save the GTD's thread here (http://www.adventuregamestudio.co.uk/forums/index.php?topic=17520.0) although I'm not sure it's relevant.
Of course if you happen to find any others, please reply in topic and I'll add them to the list.
Would it be possible to have a forum dedicated to GTD's where they can constantly be contributed to despite their age? (No, but people are welcome to post to them, or make new GTD threads in this forum and add them to this list. -Snarky) I think some of the stuff in here is still valid today and I'm sure we have some more newer experienced people now that would be willing to contribute.