Thank you very much for your two cents, heyy13!
A fresh set of eyes always help us think outside our own decision loops.
1. Water:I agree that the bottom left water tiles look the most convincing. But the thing is, they also stand out as the most clichéd (as in: overused in every cheap RPG), don't you think?
To me, the high crests (pointy and shiny at times) add to the feeling of caricatured water, close to what a child would draw if asked to represent water.
The other designs are simply not fun to look at. Their perspective and predominant horizontal lines imply that the observer is close to the surface, contradicting the game perspective. So, yeah, they're crap.

I'm still trying and I did try to come up with something else but it looks like
coral sprouting from the ground.

Maybe the pallette limitations won't let me take it further than the results I got?
I guess I'll just have to be patient and resilient to find out.
2. Sand:Yellow/gray were the only basic colours I've found to be satisfying in the portrayal of sand.
Again, maybe I need to experiment a little bit more, but it seems to me that sand can't use any lower values without looking like dirt or other non-sandy soils.
On my previous attempts, the dunes were just too big and made of too contrasting colours, thus creating too noticeable (highlighted) patterns.
It just wasn't working on the mock-up. Not knowing what else to do, I've decided to keep the yellow/gray base and spread speckles over it (
these are WIPs!), as gracefully as I could.
Maybe I'll just settle for plain colors... haven't decided yet.

Any thoughts or good examples??
I'm finding this palette/restrictions to be very interesting and educative in many aspects.
[EDIT] Adding an early mock-up to make the thread less dull.

[EDIT 02.11.11] I'm still updating the image above, as you can see (and the
complete tiles sheet link, seen on 1st post).
If you feel like critiquing any other tile, please go right ahead.
Apart from what I already said, I'm still unhappy with my grass, because I think it reads as stubble, but I haven't been able to improve it any further so far.
The trees look blobby and have too many details. Is it affecting its readability? I'll have to work on those some more.
Since this is a remake for an existing game, there's some things I just can't change, like the lack of perspective/outlines on he brick walls tiles.