You restart in True Vault Hunter Mode, the second difficulty. (Go to main menu, Select Character, re-select your character, and it should ask you which difficulty you want to play in).
TVHM is the same game, but starts at level 30, and ends at level 50. Also, some more difficult variations of enemies are introduced, like Armored Psychos. All elemental resistances and vulnerabilities are more pronounced - it is even more stupid now to shoot armor with fire, and fire does even more bonus damage to flesh. After you beat The Warrior in TVHM, entire game world (including first areas) is scaled to level 50 - this is informally known as "playthrough 2.5".
At the release of BL2, 50 was max level, so that's why Terramorphous is scaled to 50 even on Normal. Currently, there is also an Ultimate Vault Hunter Mode, the third difficulty (but I believe, you have to buy DLC to access that). UVHM spans from level 50 to level 61 or 72 (depending on DLC), and is even harder - enemies regenerate, but Slag now triples the incoming damage.