Conventions.

I think the conventions in documentaries are..
1) Use of presenter or narrator
2) Archival still photographs
3) Archival footage
4) Sense of realism
5) Public interviews (talking heads)
6) Expert Interviews (talking heads)
7) Case Studies
8) Credits
9) Rule of thirds composition
10) Non diegetic background score music
11) Cutaways
12) Sound bridging
13) Statistics
14) Quoted material
15) Animations
16) Montage editing