In previous Drupals we had a rough version of plugins -- you could load a cache plugin instead of cache.inc but that was all. The pluggability was on file level and it only existed for very few files. Drupal 7 introduced many more pluggable subsystems and now the pluggability happens on a class level (mostly). We can plug in sessions, cache, queue, field storage, password hashing... Crell is working on unifying these for quite some time now. And this is all good, however, for Drupal 8 I would like to see a lot more.