You could replace that square part at the back and replace it with more smaller thrusters.
I will, probably. But rounded many-sided engines tend to gobble up tris, so this will be the last thing I do -I prefer to save up tris for the greebling.
Try smoothing out those curved panels on the starboard side (second pic.) And for the docking ring, remember that the dockpoint has to end up flush with the hull of the dockee; having it inset like that isn't good.
Panels: check. Front dock:
not check. I intend to greeble docking clamps into the 'ring' around the dockpoint. There's got to be a way the Argo can attach itself onto other ships' docks...
The engines could use a few more sides. Bump them up to 16.
Right.
1) As has been said, the front dockport looks a bit out of place - the texture indicates a rounded part to it which I'd say would help considerably. I'd love to see some interesting greebling around it too. 
Working on it.
2) More rounded and probably chunkier engines - octagons look a bit out of place. I'd suggest something a bit more like firecracks engines in terms of shape (FS doesn't really use booster rocket type engines), but with better and more interesting recesses for the actual glow.
Right. I'll see what I can do.
3) Overall greebles and recesses - moar!
The greebles in the recesses on the starboard side are great, but the port side looks a bit empty compared to it. (This may be because you've not finished yet?) Some deeper reaching recesses and more pipe and gadget details would be great.
I'm still working on the port and bottom sides, although the bottom is nearly done. Trust me, I'll greeble it as much as I can. But I think I ought to leave a few large flat surfaces, to represent the ship's armor.

HTL Argo!
Yeah! Imagine that!

I think the dockpoints, both the sets of 2 and 3 on the left and right and the main one on the front - should be round. Other than that, though, looking good.
No. No. NO!

(j/k)
I thought so too, until I had a look at the textures on the original Argo. The dockpoint is clearly hexagonal. And though I've changed a lot of things (mainly the inclination of the dock-bearing panels on both port and starboard side), I'm still determined to stay true to the original design. I might MSmooth it a bit, to bevel the sharp edges (

) but the overall 'hexagonal-y' look will remain.
Long post, sorry.
And a couple more renders: the all-but-panel-rounding-finished starboard side and the WIP bottom.
[attachment stolen by Slimey Goober]