It is watertight, which makes a tremendous difference. Suits with this type of seam can have more panelling, meaning more flexibility, without causing more water inflow through the seams. The warm layer of insulating water near the skin stays there, instead of circulating with the cold ocean. Naturally, this type of seam is found on only the most expensive wetsuits. This tends to fall under the 'fit' category.
Most suits now have a wide, smooth seal at the neck.
Make certain there's lots of velcro to keep the neck closed. The most common type, and for a long time the only type. Check these suits for a sturdy metal zipper rather than cheap plastic, and for good, thick flaps behind the zipper to prevent cold water flushes. The major drawback of back-zip suits is the stiffness of the zipper, reducing paddling and surfing flexibility. Shoulder zippers eliminate this problem to some extent, creating new inflexibility in the upper chest region.