My major objections boil down to aesthetics and culture. The themes and layouts that shipped with Drupal 6 and below were all eye-bleedingly ugly, really obviously designed by developers with no eye for colour, layout, white space or typography. In those days, you could tell someone was looking at a Drupal website just from the sound of them rattling through the cutlery draw trying to find a spoon so they could gouge their own eyes out. This carried over to the ridiculously complex menu system and the barely usable administration methods, which made it actively unpleasant to use the system as developer OR user.

I gather D7 has improved on this a bit, though it's still nowhere near Wordpress in this regard.

The main trouble I had with the culture was that complexity was seen as something necessary for power; as in, you can't achieve a result with a simple system. You can only chop down a tree with a Swiss Army Knife; an axe is too simple. This carried over into the disastrous decision to throw out all the code, including third-party extensions, in every major version update, meaning that stuff you wrote and tricks you learned for version N would be almost completely useless for version N+1. I found that idea so totally beyond hopeless that I was forced to throw out what I had spent years laboriously learning and move over to Wordpress, which suffers from none of these problems.