It really depends on if I'm shopping for a specific project or if I'm stash building.
Sometimes I'm searching by manufacturer or fabric line, if I happen to know ahead of time exactly what I'm looking for or if I'm doing price comparisons to try to find the best deal for a large amount of a specific fabric.
USUALLY, I browse visually and prefer sites that allow me to sort by theme and/or by color. Oh, and fabric content, of course - I hate it when I can't sort out blends from 100% cotton on a site!
I love it when sites have it set up so that if I hover over a thumbnail the photo will automatically enlarge without me having to click and possibly lose my place in my search.
I am also a link-follower...if I click on something I like, I will often also click on similar or coordinating fabrics that show up.
The way I shop online is that I plunk everything I like into my shopping cart, then from there I whittle it down to what I can afford. So it's really important to me that the shopping cart have thumbnails of the fabric; otherwise I have a heck of a time doing my whittling and will sometimes give up and go elsewhere. (I am really an impulsive buyer sometimes and the impluse will often just vanish if it starts to feel like work! Maybe that's a good thing.)
I also like a "what's new" link, so I can see what's come in since last time I visited that particular store.
I like to see how many yards are in stock and whether or not it's going to be reordered - sometimes I can't get everything I want right NOW so this helps me figure out what can wait until next time and what I should snap up while I can get it.
I will almost always check out a sale or clearance link, but I get frustrated when the clearance area is one or two lonely fabrics.
Oh, and sort by price!! Sometimes I'm bargain hunting and stash building, or else just trying to fill out an order for free shipping. I always want to see if there are any treasures going for cheap!
Hope that helps.