Heroes get +25% range and +50% attack speed. Some heroes (like Benjamin) get a price discount instead. Quincy and Geraldo get an additional +50% attack speed. Adora gets camo detection and Churchill can shoot through obstacles.
Berserker Brew, Stronger Stimulant, Permanent Brew, Overclock and Ultraboost 75% discount
Ultravision: slightly more range and can shoot through obstacles (without MK)
Night Vision Goggles: can shoot through obstacles
Laser Cannon: can pop lead
Armor Piercing Darts: can pop lead
Gives a 30% discount to all Tier 5 upgrades, plus 10% discount to all other upgrades and placement costs
Heroes level up twice as fast (technically, XP cost is halved)
Scales down XPcost for heroes so they all level up at the 1x rate
Can place towers almost anywhere
Heroes give +2.5% attack speed per level, and +1.25% range per level to all towers
Heroes give +5% attack speed per level, and +2.5% range per level to all towers, as well as allow them to detect camo and pop all bloons