telltale signs of an upcoming boss battle

[Xyphon]

Earthquakes......long halls/corridors/tunnels, increase in enemy activity......

 

[G1eet]

Number 2 on that list sounds like Silent Hill 2.

I have one you can add to your list.

6) An extremely lengthy cutscene in which you hear a variety of scary noises.

Or where you hear out of place noises in game, out of combat.

 

[ajb924]

Music stops or changes drasticaly, that or a door that is damn near impossible to get open

 

[Photon987]

A conspicuously large cache of rocket launchers placed in the middle of a room.

 

[Mr Gokomatsu]

When the controller has a seizure...

 

[The Maddest March Hare]

Legend of Zelda style: When you enter a room that you just got the means to reach (e.g. Hookshot/Boomerang)

 

[ucciolord1]

....near the end of the game....

 

[Camoman]

When looking on your map, their is a BOSS FIGHT HERE sign on a particular room or passage.

 

[roboosh]

metroid does these things a lot.

The music gets quiet but builds suspense. That always gets me panicking.

 

[Quadtrix]

Paper Mario: A save block and a heart block right next to each other

 

[JayDub147]

mini-boss: when you reach the end of the only available pathway which happens to go in a completely different direction than your main objective

 

[KaiRai]

Number 2 is Resident Evil 3 all over.

You're also forgetting that it's been relatively passive, or passively easy up until said point when boss explodes out of nowhere and completely owns you in every way >_<

 

[twcblaze]

clicking on the giant door in front of you makes your character turn to your party and say "are you sure we're ready?"