Done. I'm hoping the text is okay, if not I'll change it but as I said on the site, I figured it'd be nice to have a label, and didn't want to resort to texturing randomly to fill the space; the piece already has a focus.

You don't have to use NES colours or anything like that, just for the sake of learning it's often beneficial to cut down your colour count (16 colours would be loads for a sprite like this).
I've done a quick image to show the issues of yours and highlight where Helm was going with his edit. Helms is marked "a", yours "b".
-contrast in your image is lacking, Helm added more shading and highlight in his
-your sprite doesn't read easily: is that his arm off to the side or just more armour?  How is he connected to his sword? This is more clear in Helm's edit.
-The helmet is oddly shaded and the horn has too much noise, it looks segmented and jagged.. Also, what was going on with your mouth?

That said, I like your sword more than Helms even though it uses way too many colours, the white edge is a nice contrast.
The upsized and despeckled images (very roughly) simulate how the eye will read the sprite, notice how helms use of contiguous areas of the same colour describes the form better, while yours has a lot of detail that amounts to noise in the design.

I hope this helps, what I think you need to do is focus on getting the form described more fluidly before you go crazy on the colour count: in normal art the number of colours you use doesn't really matter, but in pixel art if you use too many your art won't read like you want it to.

That said, welcome to the forums hopefully you're the kind of person that can take criticism on board and learn, because it looks like you're coming from a pretty strong foundation.

I certainly don't plan on leaving any time soon, Helm.
The half tile does help the transition (particularly under the set piece) but I'm not sold on exactly how useful it would be. I'll have to play around with that idea a little more, it's interesting to think of antialiasing at a much larger scale.

I'll get this finished tomorrow I expect, I've got to get on with getting a game from start to finished by next tuesday pretty snappily O_o

In response, before bed:
@Tourist: I think I will just polish these ones up. I don't have the time to go gallivanting off developing this for much longer, so I'll stick to the task and evaluate.

@big brother: My monitor has adjusted its colours (I'm using flux) and it shows the orange as much harsher. The green sticks out a bit more than I'd like on the brickwork too, I'm going to tweak the various forms there; the larger stone doesn't read too well, and the grass topped dirt needs tweaking.

The rockdigger crits are very valid, I'm concerned about the readability but the spec was that everything was derelict and as the machine had to double as a "method of transportation" I thought it could be a good idea to have a bit of ambiguity.

Anyway, I'll push and pull everything around tomorrow amidst a short day at uni (yes!) and a bunch of paperwork from blackberry (boo!). Thanks for the feedback, you've been a help through this regardless of  some earlier harshness.

Those certainly are 8 tiles, but I'm not sure what I'm looking at. They look so noisy. I don't think the 2x2 blocks of pixels work in this case. Lots of banding, too, which definitely won't fly in tiles so small. There are also contrast issues with the palette, which further exacerbate the readability problem. I think they fall short of the "as beautiful as possible," easily-attainable goal you mentioned earlier.

I wasn't aware that 32x32 tiles are the biggest usable size. Even just looking at this thread alone, you will find that Ptoing posted some 64x64 sized tiles.

Cheers for the criticism. I agree with you, but do understand that I wasn't talking about making the tiles as beautiful as possible, I was talking about making the clusters as beautiful as possible, which with 3 hours to work and a cartoon focus (something like this or even this), which is very feasible.
With a more detailed focus, where you're using small clusters much more often and have more clusters in total to manage, the task becomes quite a bit more difficult. I've fallen short there as well, and I'm well aware of that. I'm going to have to step up my game, surely.

However, keep in mind that aesthetic beauty is in the eye of the beholder, for example "green" on TIGSource, while I would by no means consider him an authority, posted

I love these.. Those colors as well.

If he was my client, I'd be able to simply clean up a few of the more glaring issues from what I can see (some of the banding, though I think sometimes with vegetation at this scale the banded solution is the only way to prevent it being so harsh, as well as tweaking the greys a little more) and send them off as final. If you were my client I'd be in strife, obviously, as you don't like them, but from my experience a client generally has some images at hand for a style "target", or can at least mention a few games they like the look of.
Cluster beauty comes from your placement of pixels and is a little less subjective, which is what I was aiming for.
You seem to have latched onto that phrase from the beginning and it's been bugging me, so I wanted to clear it up.

In response to your comment on 32x32 not being worst-case, that's true, but due to most of my experience being with smaller assets I'd likely turn down a job offer if it were at 64x64. Surely it'd be more practical to paint the tiles once they hit that size?

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Regardless, I'm quite keen to get some fresh inspiration. If I get time, I'll tackle this tonight, but I suppose I start counting from now. 9:20 AM 5th March 2011.

I decided to use the "Lost Civilisation generator" on seventhsanctum, as it gives you a more verbose description.

This just wide-ranging imperium was noted for its advanced tactics. It was destroyed by a neighboring country because of the people's extreme immorality, leaving behind only relics and methods of transportation.

Second click. I'm going to do 8 tiles, as before. 6 terrain tiles, 2 relics, and 1 "Set Piece" transport object which will be 64x64 at largest.
I decided to "ask for clarification" on the transport object and got

Rockdigger

so I guess the civilisation was subject to rocky terrain. The relics will probably be broken weapons and coins.

I'm not sure whether to do 32x32 or 16x16, what are your thoughts? I'm going to do some sketch design and research now, and then I'm going out for the day, so I'm anticipating this to take the weekend, possibly into monday. We'll see how it goes. I'm leaning towards the smaller size at the moment.

Off to have breakfast I do hope some other people do this, it's quite an interesting challenge.