Okay, you asked for it so I thought I'd offer a few suggestions, please don't be offended, I think it looks pretty good.
Right, here goes.... not sure how easy it would be to do with your shed at that stage of construction, but I thought busting one of the window panes and having it patched from the inside by perhaps a bit of cardboard with printing on it might look good, you'll have to be careful about period accuracy for the branding on the cardboard though!
Try a few cobwebs too perhaps by stringing glue across the corner from under the eaves of the roof (practice first though! don't ruin it on my account!!), if it's anything like my outbuildings it'll have plenty of them! I'd rust the metal up a bit too and have rust streaks from where the bolts pass through the wood.
Judging by the proximity of the grass, I'd say the lower areas of wood might be a different colour from being damp as well. Also perhaps you could add a 'hasp' type lock to the door with a padlock and/or rusty chain (cheap jewellery painted up or if it's really cheap actually rust it for real - put it in a dish of salty water). Finally, leaning something up against the shed near the door will help reinforce the scale appearance, a pickaxe, spade or rusty saw or something. Careful about the period accuracy of it though. Or why not perch a bird on the roof? Wildlife is often conspicuous by its absence in a lot of dioramas!
Hope these suggestions are of help. By the way, the rustic appearance of the wood is really great.