Tank: High Defenses, **Moderately high **sustained damage (but rather low burst), Good initiation tools, and Lock down.
The other way 'round, imo. Tanks lack huge burst, but feature decent sustained damage to punish completely ignoring them.
Examples: Amumu's W, Zac's W, Sejuani's W, Sion's W and Q (although he is a bit more bursty than the previous ones), Leona's passive, Braum's passive, Cho's E (Tank Cho, that is. AP Cho is burst galore if hitting all abilities), Nautilus' W, Rammus' passive and ult.
However, what sets tanks apart from bruisers and juggernauts is their ability to force engages with strong AoE lockdown CC. Juggernauts can feature CC, too (Darius' E, Skarner's ult, Illaoi's ult), but their CC is not made to reliably initiate a fight.