I plan to play a Bard, my class of choice in FFXI, assuming it is in the final release. If Bard is not available, i'll start off getting the hang of the game with a melee class, before shifting to a caster, trying to nestle myself into the support/healer role, but in any event I plan to try every class I can. Race is either a Miqo'Te or a Midland Hyur, and of the three starting locations, Gridania sounds interesting.
For me, the Beta provides a chance at two important things. First, the chance to see everything before the contents, mechanics, and every other point of interest within the game gets spread across the internet, before you can just go to a website to have a walkthrough on each quest, monster, and zone. It's a chance to discover, pioneer, and learn. Figuring out through toil and trial what works, and what doesn't. Spending time, gil, and effort into discovering the ins and outs of a crafting system, before 'quick and easy' methods become commonplace. Second, the Beta provides a way to make sure everything on my end, hardware wise, is enough. I'd rather loose a day or two during a Beta test, tweaking hardware and parts, and get problems in my system nailed down then, while the game is receiving the same treatment, then picking up the game at launch and finding those problems then. For me, the Beta is those two parts: the desire to pioneer, and see something fresh, in it's infancy, and to make sure i'm equipped when the real deal starts.