Some techs should be marked as untradeable - stuff like Unconventional Warfare, for example, which only exists to open up tech options for the Drengin and don't have any bonuses, modules, or improvements associated with them.
Unconventional Warfare gives +5 Weapons, it isn't worthless.
If you can list specific techs that should be made untradeable, it'd make it easier for the devs to take care of it.
Can do! Here's a list separated by categories of "shouldn't be tradeable" and race:
Definitely shouldn't be tradeable:
Many Races:Industrial Revolution, Traditional Research - These are starting techs, having these not be tradeable should delay the loss of "unique'ness" between the different AI Races. This is only listed because, if I remember correctly, a few races can trade these techs, while others can't.
Yor:Life Support - These techs are re-flavored to be maintaining their sentience, not support biological functions like the other Civ's Life Support techs.
Efficiency Studies II - The rest of the techs already are untradeable.
Drengin/Korath:Both Research Trees (Slaveling Labs and the Pain Amplification line) - These techs heavily rely on Drengin (or more specifically, their researchers) physiology. The new research methods gained from these techs would be worthless to other Civilizations. Or, at least, should be, anyway.
IconianAdvanced Weapons - These techs are supposed to be a "disadvantage" to the Iconians, a way to slow down their weapons research. Interestingly, they actually are an advantage (due to the total +15 Weapons you can easily get from them), and given that it is the Iconian's mindset that spawned these Techs, it really doesn't make much sense for them to be tradeable.
Kyrnn:
Krynniac Conversion - None of the other Krynn Influence techs are tradeable.
Ultimate Espionage - All this tech does is give access to a Galactic Achievement. Since it is far too easy to make this tech worthless to another Civ, but still get them to offer you a good bit for it, this shouldn't be tradeable. If the building was changed to be a "Super Project" I would say let it remain to be tradeable.
TorianDefensive Trace - This tech specifically refers to a part of the brain in the Torian mind (well... actually whatever Civ happens to have the tech, but that's a different point).
Scientific Method Implementation - All other races already have developed their own Scientific Method. The Torians, however, being a pre-Industrial race, haven't and are forced to quickly do so.
Probably shouldn't be tradeable:
Drengin/KorathArena techs - The later Arenas dispense with the Gladiatorial games (which might entertaining to other races) entirely in favor of just causing pain to others to please the populous (which is based on the Drengin's ability to feel the pain of others and the pleasure this feeling causes).
Yor:Synthetic Ingenuity and
Creative Insight - These are modifications to the Yor in order to replicate functions already present in Biological Entities.
Iconian:The Organic techs - The rest of the Precursor technology the Iconians get isn't tradeable (barring Xeno Persuasion, which makes sense to able to be traded, to me anyway) and these techs are the Iconian's main military strength.
Shouldn't be tradeable, but just IMHO
All RacesLife Support Techs
Colonization Techs
Farming Techs
All of those techs heavily rely on the physiology of the race in question. Granted, one could argue that when these techs are traded, the races simply apply and adapt the techniques to match their unique Physiology, but I would like that their be some things that you just can't Trade for techs to improve upon (Range, Colonization, and Farming being my choices).
Other Suggestions
I suggested this elsewhere, but there (IMO) really needs to a third level of "trade-ability" - Tradeable only if both races have the tech in their tech tree. Two races with the Yor tech tree, for example, can't trade their Morale, Farming, Economy techs with each other like the races with the more standard Tech Trees can.
There are also a few techs that aren't Tradeable that I feel there is no reason they couldn't be tradeable. A few of the Scatter Blaster and other Weapon techs in the Yor tree are an example. If I get inspired enough, I might list them out later...
Other Tech Tree Issues:The Thalan Tech tree has the "IndustrialRevolution" tech in it, but it is not possible to research it. I doubt it was intended to actually be there, though.
About the Iconians tech tree... the "Category" tags' contents seem extremely out of whack. "Missile" for the Industry tree? "Logistics" for the Economy? "Beam" for Advanced Mass Drivers, "Ethics" for Organic Hulls, and even "Trade" for Advanced Beam Weapons?!! I dunno what happened here, but it's no wonder to me that the Iconians never seemed to do very good when they were an AI player in the games I played... at least if I understand the use of the "Category" tags correctly (that they help the AI choose what to research next based on what it thinks it needs to improve on).