Well there are several things that could be done to improve tech trading, which to me makes more sense that doing away with a core mechanic, somewhat down to a race's personality perhaps
1) The player should get strongly labelled for trading techs to a race's opponents, it should carry a significant diplomatic malus for a fair period of time, making you pick and choose who you trade with carefully, meaning the decision carries weight.
2) Having set random techs that a civ decides, at the start of the game, that they won't trade AND/OR 3
3) More useful techs that the race decides have a higher trade weight - Goes to personality/strategy, I mention this because of the below:
4) Techs it values less because everyone already has them. - Goes to balance.
5) Techs it values more because too many people have them, and so won't trade them. - Goes to exclusivity
6) Stronger alliances required by some races before tech trading. Some races, seems perfectly normal others would be open with their research.
7) Broader tech tree as the game expands, means there is more to research, and so a slower spread when coupled with:
8) Max amount of techs many races will want to trade with you in a period of time or ages, again different personalities should have different limits.