In the history of Tunnels & Trolls, the biggest evolution in the game was the jump from 4e to 5e. I'm sure if I sat down with my copies of both editions in front of me, I could turn out a fairly comprehensive list of the changes, I'm also sure I'd miss quite a bit. Instead, I'll turn to my recently acquired copy of issue #5 of the Sorcerer's Apprentice, which has a fine article by Liz Danforth that describes, in detail, the changes. I'll be foregoing most of the fine details ;)
1 - Rogues no longer have to choose between Warrior or Wizard at 7th level. They do stop progressing in magic at 7th level in 5e. No use of magic tools, no lowering the cost of casting and no inventing new spells.
2 - Warriors now get 2x the listed armor and shield protection. Finally a reason to play a warrior.
3 - Wizards limited to 2 dice or less weapons but gets combat adds from str, luck and dex
4 - Kindreds - changes to rolling elves, dwarves and leprechauns
5 - Experience is now Adventure Points. Gold no longer gives expo / AP.
6 - Magic - Only wizards can buy spells directly out of the book, others need to be taught them. Bunch of new spells.
7 - Charts - New weapon, equipment, leveling, poison, armor charts.
8 - Monsters can now have a negative charisma
9 - Missile Weapons - new system
10 - Speed as an attribute is introduced.
I forgot to mention the Warrior-Wizard class, but who can actually qualify for that rolling 3d6 for their stats? heh
I'll do up the 5e to 5.5e changes relatively soonish.