I think this new version is phenomenal compared to 1.4.x and all predecessors. Fantastic work! Just a couple of minor suggestions:
1. Rename the 'Basket' button in the primary navigation to 'Shopping Cart' or 'Cart' for consistency with other pages/links. Most people won't be looking for the word 'Basket' (unless, of course, the entire site uses that prominently).
2. The 'Shopping Cart' popup box at the top (next to Search) should include a button or text link 'Go to cart'. There's space to the left of the Checkout button so it'd be a handy shortcut. You could also add 'Clear cart contents' but that might not be so important.
3. The payment options should be reinstated to the bottom left in the footer. Firstly, it's reassuring for buyers to see those logos there, especially if the store owner adds a security company logo. Secondly, it helps people earn PayPal referral commissions
4. For your own promotional purposes, I'd insert the OpenCart logo into the bottom right corner rather than just the text link. Store owners can still remove it if they wish.
5. Does the site map need to list all categories and subcategories? I know this is for SEO purposes, but stores with hundreds of category/subcategory combos will have a super long Site Map page. Could it simply show top-level product categories only?
6. I think the 'Login' button is redundant. If the user is not logged in, make the 'Account' button read 'Login'. If they are logged in, have it read 'Account'. Then add a text 'Logout' button somewhere (e.g. to the left of 'Home'). Just trying to save some screen real estate and less items in the primary navigation.