Cross browser compatibility

Make sure you check that all CSS/HTML renders correctly and consistently across IE7-9, Firefox, Chrome, and Safari. Make sure the site is functional in ie6 - it doesn't have to be aesthetically perfect.

Style common tags

Uploaded images should not break your layout

Use the asset URL when pointing to any image in your stylesheets

                background: url( {{'background.jpg' | asset_url }}) 
              

User can upload a decently sized logo

Nav doesn't break when too many links are added

Your linklists/navigations shouldn't break the layout if the user adds too many links.

Theme can handle different product aspect ratios

Avoid fixed limits

No dead ends

All templates are styled

Compare price is enabled

Covers all product combinations

                {% if product.variants.size > 1  %}
                <em>There are more than 1 variants, let's show dropdown(s)</em>  

                {% else %}      
                <em>If there is only one variant, let's use a hidden input field  </em>

                {% endif %}
              

Removable attributions if it's a paid theme

Attribute Shopify

Additional checkout buttons code snippet is there

                {% if additional_checkout_buttons %}
                  {{ content_for_additional_checkout_buttons }}
                {% endif %}
              

Checkout pages are styled and show the shop logo when appropriate

Your theme supports customer accounts

                "customer_layout": "theme",
              

Your theme contains social sharing buttons and links

Your theme can display methods of payment

Nav doesn't break when too many links are added

When a customer installs your theme they should not see any blank areas or be confused on how they can populate certain areas. If your theme relies on the user creating a linklist, page, collections then make sure your theme instructs them to create supplementary content.

Don't use an image logo - use the shop.name text instead

When a user first installs a theme - they shouldn't see the demo store logo, but the store's shop.name instead.

Can a new user use the theme?

Always keep in mind that the user is probably brand new to Shopify so make sure your theme is as easy to customize as possible.

Only use existing store data

If your theme relies on the user creating additional content (like a new collection) then please only use the default Shopify content when a user first installs a theme. For example if I give the user the option to include 2 pages on their homepage my index.liquid might look something like this...

                {% if settings.frontpage_right and settings.frontpage_left != empty %}
                  <ul class="two-pages clearfix">
                  <li>{{ pages[settings.col_left].content}}</li>
                  <li>{{ pages[settings.col_right].content}}</li>
                  </ul>
                {% endif %}
              

                <tr>
                  <td><label for="col_left">What is left column page?</label></td>
                  <td><select class="page" name="col_left" /></td>
                </tr>

                <tr>
                  <td><label for="col_right">What is right column page?</label></td>
                  <td><select class="page" name="col_right" /></td>
                </tr>
              

Notice how I picked the welcome page and the about us page - both of which come with the default Shopify install.