This is pulled from the Web Accessibility in Mind website:
"Trebuchet is an attractive font, but it has subtle curved embellishments that may decrease overall readability for long passages of text. The curve at the bottom of the lower-case "L" helps to distinguish the "L" from the upper-case "I," but when the "I" is viewed out of context, it looks like it could be a lower-case "L." Some of the glyphs are also presented in a non-standard, more decorative format, such as the ampersand (&), which may also decrease readability."
"This is still a rather popular web font because it "has some style," so to speak. It is unique and has an artistic feel to it, but is still readable for the most part. In terms of accessibility, it is better than some fonts, but not as good as others."
2. Which fonts should I be testing with?
Penn State has some recommendations:
Highly Recommended Fonts
Verdana: Designed for monitors by Microsoft. Many sites on accessibility use Verdana.
Lucida Sans (PC)/Lucida Grande (Mac): Relatively new font. Used as a Mac system font.
Tahoma: Available from Microsoft
Georgia Serif: Designed for monitors by Microsoft.
Palatino (Mac)/Palatino Linotype (PC)/Book Antiqua (PC) Serif: Traditional print font. Weight can be light.
Andika: A free sans-serif font from SIL designed for beginning readers with clear letter forms and foreign language support. May also be suitable for readers with some reading disorders.