This tutorial will guide you through all the steps required to replicate the Commerce Marketplace demo site on your own server, and then explain how its behaviour differs depending on various configuration settings.
By default Drupal gets installed with only one default language - English - and all additional languages need to be added afterwards, followed by downloading and importing all relevant interface translations.
Working recently on a client's project it turned out that there is no good way of indexing Entity Translation-based multilingual content in solr using Search API in combination with Search API Solr search.
While Drupal for Facebook module seems to be the best and most flexible option out there for integrating Facebook with your Drupal website, one thing about it that ticks me off is its forced single sign-on approach between Facebook and Drupal.
Although Drupal 7+ runs smoothly on PHP 5.3, Drupal 6 still feels much better with PHP 5.2. Therefore developing for both D7 and D6 at the same time becomes much less painful when running both versions of PHP in parallel.
One of the new things introduced in Drupal 7 as compared to D6 is utilization of modified Seven as default administration theme, which - combined with new Overlay module added to core - brings completely new experience to admin pages.