You can use your dining table to pick up on existing shapes in your room that you want to draw attention to. A square or rectangular table, for example, will highlight the other rectangular elements in a room, from art work and cornicing, to large windows or kitchen cabinetry.
Likewise, a round design can be used effectively to soften a space with a lot of angular cabinetry or furniture, or one that's painted in a bold shade. By choosing a circular table, in natural oak, say, you can 'dial up' the softness of the overall look.
And if you'd rather keep a few points of difference in your space, rather than the continuation that comes from choosing a coordinating shape, choose a table with contrast.