Too many words on a slide and too small a font (contrast Lawrence Lessig's slides [0] or Dick Hardt's [1]). Looping animated gifs (especially when they're overused). Bad fonts, colors, layout. Monotonous delivery (e.g. saying ”I'm excited” without excitement in your voice).

NOTE, I just sat through 2 days of slides, so I'm a little cranky.

[0] https://youtu.be/rHTBQCpNm5o

[1] https://youtu.be/RrpajcAgR1E