The edges of MDF are fragile, easily crushed or torn. MDF is also notorious for absorbing water through these edges, causing the panels to swell. In Sam Allen's original design, he edged the MDF with 1/4" hardboard. This edging is one of the complexities that Asa Christiana left out in his simplified design. I think this was a mistake. MDF really needs some sort of protection, especially on the edges.
Drill a pair of 3/8" holes in each end of the short stretchers, just over half of the depth of the dowels, using a brad-pointed bit. These stretchers already have a groove running their length, centered on the bottom edge. Precise placement isn't necessary, but keeping track of which part is which is. We need a hole in each end of each stretcher. Take care to keep these holes square, you don't want them running at angles.
The top is supported by the top ends of the legs and the top sides of the top stretchers. Stand your legs on end on a flat surface (like my door) and see if they wobble. If you have an end that isn't quite stable, use it as a foot, where the leveler will make it's flaws unimportant. Check the top edge of each stretcher for straightness. If one has a bit of a bow, use it for a lower stretcher. It's less critical that the shelf be well supported along its length.
Line is defined by two points and is long relative to its width; it can be thick, thin, vertical, diagonal, straight or curved. Lines are often used to define a space, draw attention to a particular area and guide the viewer's eyes around a piece. To critically examine line in your design, look at the relationship between the lines, including ones which may be created in the negative space. How do they align? Do they lead your eye around the piece or do they stop abruptly and create disorientation?
Grizzly is a company currently undergoing an upheaval. This brand was once known as a manufacturer of professional grade tools and equipment for woodworking products. Though, in recent years, that reputation has begun to slip a bit since being bought by the Chinese company SIEG. These days, whether the Grizzly product stands up to its previous standards of excellence often has more to do with what you are buying than the company itself.