Blizzard plans to remove the ability to pickup siege-tanks; and they play to buff the siege tank damage; this may change any information below.(2/18/2016)
Marine-Tank is currently the mid-late game meta.
Progressively making your composition heavier in tanks.
Some people add in liberators later in the game to move tanks out of position.
Watch out for: Early game tank drops, 2/3Rax-Reaper rushes, Cloaked-Banshees, Early Hellion/Reaper pressure.
Marine-Tank is also really popular; it's wise to open with a tank rather than hellions - b/c of roach-ravager pushes and banking busts.
Some people play pure marine tank in the early-mid game (no marauders). Though you should be adding in marauders if they go heavier in roach rather than ravager.
Mid-Game add in Liberators; they'll be essential vs Ultras. You can also add in Thors and Ghosts(snipe) to kill Ultras.
Expect to see Ultras at around the 10 min mark; bio is practically useless against it so you need to be prepared.
It's a free win if they go for Lurkers. Tanks outrange them. Just don't take bad engagements.
Watch out for: BaneBusts, Roach-Ravager pushes, Overlord drops, Nydus allins (not as popular as it used to be), late-game Ultras.
Marine-Tank/Mine-Marauder is the most common composition.
Protoss could be going for either Storm, Disruptors, or Colossus(becoming more popular)
Adding in Liberators helps a lot... Definitely an essential.
Previous patch; Pylon-Overcharge was "nerfed" (no longer spammable) so drops are a-lot more viable, so is early aggression.
Watch out for: 4Gate, DT-Rush&Drop, Chargelot allin, Proxy Robo (immortal allin), Pylon-Overcharge Rush (less common b/c of last patch), Warp-Prism harass(can be game ending)