So, Medics and Goliaths aren't in the SC2 multiplayer. Having played Broodwars before I can understand your disappointment. However, that doesn't mean Terran is under-powered.
Medics had an important role in the original Broodwar meta-game of helping you survive until you hit Factory Tech (or allowing +1 Bio timings). As a unit, they functioned to give Marines extra needed beef so they could survive Zergling/Hydralisk and Zealot/Dragoon. In SC2, this roll is replaced with the Marauder. The Marauder is an Armored unit with 125 HP and 1 Armor. This makes it ideal for dealing with fast attacking Zerglings and Zealots. In addition, it does double damage against Stalkers (Dragoon replacement) and Roaches (Hydralisk replacement as they were moved to Tier 2). The flip side is that they do very little cost effective DPS to Zerglings and Zealots (though they do have an upgrade which slows those units to make them easy prey for Marines). This allows for strong Marine/Marauder Bio pushes (usually off of 3 Barracks). Additionally, Marines have an upgrade called Combat Shield which give them an extra 10hp of Beef.
Between Combat Shield and Marauders, Blizzard didn't feel that Marines needed more beef in the tier 1 arena. However, because of the mechanic of Stim (same ability, but also applies to Marauders), they did add in the Medic as a tier 3 unit. Although, at tier 3 a simple Medic was too little, so they instead merged this unit with Dropships to form the Medivac. This unit allows the weaker Bio army to continue to function well into the late game.
Goliaths were another unit which was cut from SC2. While Marines still serve as a decent anti-air platform, they had neither the Range nor the heft of the Goliath. To compliment Marines, Thor's were added for Terran at the tier 3 level. The simplest explanation for Thors are that they're two Goliaths merged into one: twice the HP, twice the DPS, twice the cost (and twice the size!). However, because they are a tier 3 unit (and expensive) a second anti-air unit was added to the Terran army, the Viking. Vikings largely replace the Valkyrie in the tech tree, but function more similar to Goliaths than Valkyrie. Long Range (10), Single Target Anti-Air attacks that were once the trademark of the Goliath can now be found on this unit. In addition, it had the ability to transform into a land walker for ground to ground attack (providing additional harassment opportunities). While the Viking largely replaces the role of the Goliath, the Thor (strangely enough) gained the AoE of the Valkyrie, in addition to an impressive ground to ground attack (45x2). This allows them to function as a backbone for many late game Terran armies.
Things have changed between SC1 and SC2, but that doesn't mean that those roles have disappeared. With a little practice (and maybe a few VoDs), you'll start to pick up the differences between the two games.
IdrA(it's hilarious).