One feedback that I received recently was related to images consistency and coherence.
You shouldn’t mix and match different styles, clipart, stock photos, images taken from the internet, created with different resolutions, sharpness, grain, etc.
When you’re creating the slides, you probably don’t notice too much difference.
When the slides are projected on a big wall, or, even worse, when the slides are viewed remotely on a big monitor in front of people’s faces, they’ll immediately feel the differences, even without noticing.
I’m not saying that you couldn’t use different styles in different sections, but always try to be consistent, at least in consecutive slides.