If you will be using it for a production type environment I would probably put evenly spaced holes for clamping but otherwise I'm not a fan of holes or slats. I prefer a flat smooth table top and if/when I need a special locating tab for something it is nothing to tack weld one onto the table top then when done knock it off and flatten the tack with a flap disk.
I also like some overhang so I can sit up under it when welding as well as for clamping purposes. Mine has a 6" overhang.
One thing I am adamant about when talking about fabrication tables is overall height. I see too many that make them as tall as a standard work bench. I prefer my fabrication table to be slightly lower for two reasons. First so when TIG welding I can sit at a normal sitting height and not feel uncomfortable, usually 30"-32" to work surface. Second, if you have anything of any size on a tall fabrication/welding table then you have to get a step stool to reach the top. When I was fabricating ATV sand drag chassis they were in the 16"-24" height range and on my 32" I could still easily reach the top rail of the chassis to weld on while standing on the floor.