Twitter API can be a major PITA sometimes, but luckily there are modules for Drupal that makes integrating it to your website easy.
Sometimes you just need to get your hands dirty and start adding Drupal Commerce products programmatically. Luckily that is not that hard of an thing to do.
Drupal has a block called "Language Switcher", which displays links to different language versions of the current page/node. If the node does not have translated version on the specified language, the block will not display a link for it. This can cause some confusion, since the user always expects to see links to all of the site's languages.
This can be fixed by modifying the block so that the all of the links link to the corresponding language's frontpage. It is easy to do by editing the site's theme.
If you have your product categories-vocabulary in a menu, Drupal Commerce can't by default activate the active-trail when viewing a product node. This can be bad UX, since user has to remember from which category he came to the current product.
One of the first things I usually need to do when installing a new Drupal site, is to change the settings in the new (version>4) CKEditor so that I can type classes and ids and not have them strip out.
When migrating your Drupal 6 to Drupal 7, one of the most important issues is the theme. Luckily, the base of the themes in Drupal 7 haven't changed that much, so the conversion is pretty straight-forward. Here are some tips for converting your theme, divided on different files in the theme.