Wow the D section is tough. I've completed all of the levels except for D6 and D7 (D4 fried my brain) and I really like the level design. It's tough to count the distance between islands sometimes but there's only been a few levels where that's been an issue.
edit: I just beat D6 and D7. D6 especially looked like it was going to be a little messy when I started but it was actually very tightly designed. Great game!