The 2 things that really prevent me from ever fully switching over to Android:
- iMessage - Yeah I know this is common, but literally 90% of my friends, family, and coworkers have iPhones.  Every time I switch to an Android phone, this becomes an issue.  SMS is just not comparable.  Whether it be seeing if a message is being replied to, or uncompressed photos/videos, the messaging experience on Android is not a good experience compared to iPhone.  And it's a big deal because it's the app I use the most.
- Apple Music - Yes I know it's available on Android, but the integration with Siri/Apple CarPlay is just fantastic.  With music being such an important thing to me, I want the absolute best and user experience possible.