Have you planned for storage space for bulky items like long lengths of fabrics and batting?
And storage space for the itty bitty things - like scraps, and pins and needles?
24 x 24 gets smaller and smaller after all the goodies get put in place.
Pets can be good company while sewing - they can also be a major nuisance.
People have already mentioned lighting -
if you do go with carpeting, a short even pile works fairly well - I have that in our basement - pins really don't sink into it - but maybe that's because it's pretty well worn now. :?
Hard, smooth surfaces would probably be easier to maintain - I would want a cushioned mat by the ironing/pressing and cutting areas.
Do you need to go outside to get to this space?