Stuff like, "I want to draft a black aggro deck so gravecrawler looks good, hey look all of my other black aggro beaters happen to be zombies." What I think of as a real tribal deck, is a deck where each addition of the tribe to the board is building up this interlocking engine that eventually just crushes the opponent
Alistair, the Brigadier Artificer's Assistant Basim Ibn Ishaq Cabal Paladin D'Avenant Trapper Daring Archaeologist Diligent Excavator Elena, Turk Recruit Glin, Dwarf Emissary Jamie McCrimmon Jhoira, Weatherlight Captain Lingering Phantom Mishra's Self-Replicator Moira and Teshar Sarah Jane Smith Serra Disciple Surtr, Fiery Jtun Teshar, Ancestor's Apostle The Sixth Doctor Traxos, Scourge of Kroog Traxos, Scourge of Kroog
The same idea applies to cards like Shilgengar, Sire of Famine
On the other hand, it is readily beaten by many Flying-types in the game
If you turn Lair of the Hydra into a creature but haven't controlled it continuously since your most recent turn began, you won't be able to activate its mana ability or attack with it