I thought the NPCs in Wind Waker were fine. Windfall Island had plenty of interesting characters, the pirates were really cool, and all the other characters and races you met throughout the world (Deku Tree and the Ritos and all) were great. I felt they were better than Ocarina of Time, in terms of good interaction, but paled slightly in comparison to Majora's Mask. But it relied very, very heavily on NPC interaction, whereas Wind Waker was a cross between that and adventure.
Of course, if you only played the main game there's a lot less interaction. If you only do what you need to, dungeon dungeon dungeon. But I did pretty much every side quest and extra bit available, so I witnessed tons of good NPC relationships. I also felt many of them were much more sincere than previous game had been. I felt more for Aryll and the two Rito kids than I ever felt for most other characters. Except for maybe Saria and Marin, but that's hard to beat.
As for being lonely on the see. . . yeah, at times, but appropriately so. I visited characters here and there often enough that I never felt truly isolated except in places like dungeons.
I think this game could be positively lonely in a lot of places. But I feel there should be NPCs, and probably will be, I'm just not sure how everything will work out.
I say that Zelda is about adventure, plain and simple. Sometimes it's the childhood sense of it (and not the corny 90s movie sort, either) and sometimes it's more general. It's just always given me that awesome feeling of adventure, and I find that summing it up in that one word works fairly well for me.