Marines excel against immortals, given their vulnerability to low-damage units. However, they are weak against the colossi, due to splash damage from their thermal lances and the high templar's area-of-effect psionic storm. Marines are also ineffective in small numbers against heavily armored units such as thors, ultralisks, carriers, and battlecruisers. Groups of siege tanks in siege mode can also easily wipe out large numbers of marines far from the latter's range while hellions and hellbats can do significant damage to marines due to their bonus to light units. Lurkers and banelings will also beat marines.
M98 Combat Shields are a vital upgrade for any marine fighting on the front line, 30mm of bonded carbide may not sound like much, but field tests have shown a full 25% increase in marine suitability when equipped with Combat shields.
Attack upgrades improve attack cooldown of affected units by 5% per level.
These self-replicating ultra capacitors reduce the reload time of weapons and systems. Now each of the weapon upgrades at the armory and engineering bay increases attack speed by 5% in addition to increasing damage.
Armor upgrades increase health of affected units by 5% per level.
We have created a new lightweight alloy called vanadium. Vanadium diffuses weapon impacts much more efficiently than traditional armor plating, and it better preserves the life of our units. Now each of the armor upgrades in the armory and engineering bay increases unit life by 5% in addition to increasing armor.
Early in development, the the StarCraft II team experimented with changing the base tier 1 units for each race. This included replacing the frontline role of marines with the prison infantry guard, a much lighter armored terran infantry variant. However, this idea never made it past the concept art stage.
Concept art exists for marine variants in StarCraft II (or possibly as separate units). These include a demolitions veteran, an anti-air veteran, and a heavy artillery veteran. Art likewise depicts marines equipped with a wrist cannon (or similar weapon) rather than the C-14 rifle.
At BlizzCon 2007, marines had a bayonet attached to their rifles but these disappeared from later versions of their model.
↑Yup, the Marines still have upgradable shields, making a huge difference late game against more high damage units. As long as those Marines can survive that initial hit, the Medivac Dropships does an awesome job of bringing them back to full health in no time. Karune. 2008-09-16. What happened with the marines and shields?Battle.net StarCraft II General Discussion Forum. Accessed 2008-09-16.