I guess I'll start by saying that unless you had a specific reason for using 24fps, it's probably not worth bothering with at the moment. It's just too tough to keep the animation from looking spastic, and the end result tends to be no better than 15fps anyway.
As far as the walk cycle goes, the most obvious problem I'm seeing is the back movement. If you try to keep the character's back at a consistent angle through the whole walk, it should look much cleaner. I can't quite see the individual frames, so I'm afraid I can't really discern if there are serious issues with the legs. As long as you're using around 5 frames for one step, you should be good there. The arms are the last thing I would address- it looks like you are moving them counter to the motion of the legs, which is great, but there's a pretty large movement between frames. There are three solutions I'm seeing-
1. cushion more by taking an extra intermediate frame between the beginning of a step, the middle frame, and the end of a step.
2. take the same number of frames but move the arms less so the difference between images isn't so extreme.
3. take the same number of frames but move the arms more so it looks intentionally jerky. This would be appropriate if you were looking for comedic effect.
For what it's worth, I really like the run-cycle. It reminds me a bit of Crispy-Cheeser. There's not much in the way of smooth run-cycles for LEGO, so I think what you have right now is workable as far as the actual motion. You might want to add flourishes like skidding on turns and such.
