At higher levels all kung fu can appear very similar and seeks the same things.

Many or most of the traditional northern systems are older. Before switching to Choy Lee Fut in '93, I had trained in N. Praying Mantis mostly, and some Cha Chuan (Zha Quan) since 1985. So these observations are based only on my own experiences:

Many southern systems' sets (forms) are performed in a cross pattern (front-back and left-right). Most northern sets tend to move in a linear pattern either left and right or just front and back, although some systems incorporate complex circular turns as well.

The movements of many northern systems' sets (not all!) seem to be mostly right-side dominant, whereas many southern systems' sets seem more right-left balanced (there are exceptions to this!).

Hard power seems to be taught sooner in southern style, though both CLF and N. Praying Mantis (NPM) stress the importance of developing relaxed power. My CLF training horses are lower and wider than what we learned in NPM. Speed of footwork in fighting drills, however, is roughly equal.

In NPM, we learned in-fighting much sooner. In CLF we started with long-range tactics, and as we progress, we achieve the close-quarter skills. The NPM is still more relaxed in this aspect.

Both NPM and CLF are aggressive fighting systems, but CLF contains more hard, "smash-through" tenderizer-type attacks to sometimes break through the guard or open up oppenent. CLF also seems more suited to multiple-attack situations, though it's just my impression.

Oddly enough, in fighting theory, we use more kicks in CLF than NPM, but still not as primary, but as backup.
Jim