Nevada - I was just about to go searching for calibration threads on this. Mind sharing some settings?
Which 24" did you have? Maybe something is up with my 27" that I don't find it brighter than my 3 year old.
My 24" was (is) Core 2 Duo 2.8GHz. Check your Display icon in Settings and make sure you have the slider set all the way to the right for max brightness. Also double check your Energy savings settings... the screen will dim before going to sleep. I am no techie so am just reaching to help at this point.
If you get stumped call Apple Care... they are really helpful and will guide you to identify a problem if there is one. I had a problem with SL loading (system crashed) and they spent 2 hours with me on the phone working it out.
You can increase or decrease the size of the font in Mail by hitting Command and + for larger and - for smaller. I can't find a way to increase the size for the listing of incoming mail yet but am hunting.