I used to have a Mac Mini server, used for backups, photos, and media management (Plex and others). However, I sold it and got a DS1815+ and it's waayyy better.
The obvious - more bays, more storage, more storage options. I have 5x4TB set up in SHR2, so about 12TB of space. I bought it used on ebay, and it came with cheapo drives, but so far so good.
For media, I stopped using Plex, and just host the files on the NAS and play them on my AppleTV using Infuse. It seems the hardware transcoding on an ATV is better than the software transcoding in Plex. Of course, this means the network connection has to be solid as a 4k movie can require up to 128 Mbit/s in speed to stream with no compression. After trying both SMB and NFS back and forth, I think the current implementation of NFS on Infuse is a tad faster, so that's what I use.
Recently started playing around with Docker as well - the possibilities there are literally endless!
I have only messed around with Surveillance Station for a few minutes, but it seems like a pretty robust option for security recording.
I have a buddy with a nearly identical Synology NAS - my goal in the next few months is to put one drive in his NAS to use as an offsite backup to my most important files.