Quote from 0_Zippy
Overall, it all sounds good, but to be slightly more specific...
Potion of Immunity: Good
Potion of Vulnerability: Genius
Potion of Cooling: Could be solved by a water bucket.
Potion of Scorching: Enchantments and Flint & Steel already do this.
Potion of Luck: Genius
Potion of Cursing: Luck potion already covers this, but seems useful in Splash form.
Unbreaking (Swords): Fair
Leech: A tad overpowered in some cases, but not bad.
Smelting: Hurts the point of furnaces, since one can get XP from furnaces.
Knockback Resistance: Genius
Unbreaking (Shears): Not heavily needed, but fair.
Extracting: Not heavily needed, but good.
If you want icons for all of those, I'll give them to you.
Sure! Thanks for the feedback and all those 'Geniuses'. Those made me feel really good.
About Potion of Cooling. Some servers that have PvP arenas do not allow the placement of water from a water bucket, so this would be a good alternative. It also doesn't make much of a mess a regular water bucket does.
Potion of Scorching time. Some servers also prevent the usage of Flint and Steel, so this would also serve as a good alternative, and good to hit a massive group of mobs/players, not shooting them one by one or hitting them one by one.