Why it's good: (1) The semi-transparent overlays on the left help the text stand out. And since the overlays are colored to match the background, they don’t look too stark. (2) The colors in the design match the logo, which helps tie the two pieces together. (3) Artfully-done gradient adds some perspective to this flat design.
Why it's good: (1) The text colors alternate in a way that makes sense – you could just read the red text (or vice a versa) and get the gist of what this poster is about. (2) The most important information (when and where) is located at the top, in black text (easier to read than red), and with decent spacing and differentiation in text size.
Why it's good: (1) The time and location text is large, making this an easy design to scan. (2) The white background clearly segments these blocks of text and helps lighten the right side of the design. (3) The different colors on the presenters help keep the text box from becoming a non-descript blob.