Before I start, let’s take a look at what appears to be the problem and how Blizzard is attempting to solve it. The problem is essentially the phases of the matchup. In the early game, the Terran has a powerful advantage in terms of
 

-Zerg is unable to scout the Terran effectively or without cost

-The Terran has a multitude of openings possible from the same build order or slight changes which are difficult to see

-Gas is very limited for Zerg in terms of unit cost, making access to powerful tech units difficult
-Terran has better early game army efficiency

-Money is limited for Zerg due to larvae constraints by units and workers

-Terran units have very strong harassing properties such as Hellions, Reapers and Banshees.

That is a lot of Terran openings against Zerg

Now once the midgame appears, Zerg will generally have the advantage. This is due to

-Zerg has enough or more workers than Terran

-Zerg tech is finally available

-Zerg has acquired additional gas at the second base

-Zerg reacquires map control through tech units

Terrans have three ways to go about in Terran vs Zerg.

-Pure Biological Units (Marine Marauder with Medivacs)

-Biomech (Marine Marauder Medivac with a few tanks)

-Pure Mechanical (Hellion, Tanks, Thor)

Each of these pathways has their strengths and also their weaknesses. The Pure Biological army is good because it has high mobility and damage per second. Secondly, it is a self regenerating army provided medivacs are in sufficient numbers with sufficient energy. It is capable of being entirely offensive or defensive with no terrain constraints with use of Medivacs which also acts as dropships. The Biomech is strong for being much more capable of holding static terrain with the use of Siege tanks. It also has long range while being able to move flexibly with biological units. Mech is pure toughness. It can take a serious lot of beating with units such as Thors and capable of a strong defense under relentless pressure.

All three options at this midgame stage will usually be unable to move out effectively onto the map without getting killed. This would give the Zerg map control and expand uncontested. Why is this so?

This are the problems encountered by Terran provided the Zerg performs the correct response.

Pure Biological units

Banelings with speed upgrade can completely annihilate the Terran Army. Then some remnants of Zerg tech units such as roaches or mutalisk would be able to the minimum damage Terran even more. In the worst case, they maintain map control and gain accessibility to expansions and flanking positions. Additionally with the use of infestors, the ability of Fungal Growth can cause damage and immobilize entire armies if placed properly. This forces Terran Medivacs to spend energy and nullifies their mobility which essentially nullifies two of the core advantages of a bio based army.

Pure Mech

At low numbers during the early game, Mech is not very strong. It requires a lot of units in order to have the exponential strengths of the Mechanized army. Thors are not as efficient against mass mutalisks due to un-stacking micro. This forces Terran to be defensive with complement of turrets. They are also very inefficient to dealing with Broodlords. Additionally, siege tanks aren’t very effective in tank mode in large army battles. The requirement to siege forces Terran to remain static and immobile.

Biomech

Biomech has both the strengths of mech and biological armies but also both weaknesses. Banelings with the necessity to siege and remain static causes a conflict of synergy despite the combination of strengths.

Banelings, the Bane of Marines

Banelings go kaboom