Don't over-weld the table with welds bigger than needed or welding 100% of joints. It is easy to distort the top and many people are so excited to weld up their table they way over do it.. Less is more. It is just a table and doesn't need much weld.
Think through where you are going to be working, standing or sitting the most. You don't want a cross-brace, shelf, or hanger, right in your way when you want to work right up against the bench or try to clamp something down. A shelf on part of the table is nice for storing jigs, shim pieces, etc..
Layout where clamps, grinders, tools, are going to go on the bench before you cut or weld anything.
You might want to put some drilled and tapped holes or slots for hold-down clamps closer to the center of the table where clamps from the edge can't reach.
Make it easy to move. I know you got castors and that is great.
Don't remove the mill scale on the plate. The mill scale helps keep spatter from sticking to the table for awhile at least.
