If you're seriously interested,
read this blog/wiki/ramble about the major differences. It's very well written, and was one of the main reasons I dipped my toes properly into BSD in the first place. After a day of 'culture shock' moving from a couple of decades of Linux, I was seriously impressed. BSD is much more centralised and concurrently developed than Linux, and (imho) has a 'better' licence. It also traces its ancestry directly back to Unix software from the 60s, so it has real 'pedigree'. Linux is a (now rather large) kernel. Each distro takes a 'snapshot' of the kernel and then grabs lots of different userland apps from lots of different sources - everything from openSSH to Firefox - and 'tacks' it on to the kernel. Each distro is, essentially, one big jumble of software from thousands of different places to make it usable (even from the CLI/server).
BSD on the other hand, is developed as-is and in house as a complete 'core' OS. You can still add tens of thousands of things on top (pretty much almost everything available for Linux, and more), and it's even more flexible than Linux in that regard. However, you can download the latest FreeBSD (for example), install it, and you have a whole OS ready to deploy and use. It will be able to handle everything from file storage through to 100Gbps+ file serving or running an ISP (and many companies - including Netflix) use it for just that.
If you downloaded the Linux kernel you've got... well... a kernel. Good luck with that.
The 'consolidated' nature of BSD makes it inherently more stable, secure and reliable. For example in the history of OpenBSD there has been a grand total of two remote exploits in its entire multi-decade history. Two! Most 'modern' and commercial OS (heck, even just single programs) have many more than that a week! BSD also has a very nice easy flow to it and the code is very easy to read and understand. You can learn most of what you need to know in less than a few hours if you already have Linux experience. The online documentation (and man pages) are far superior, and the community is a heady mix of sysadmins, engineers, academics and others who use the system professionally and also happen to develop for it. It's a great community to learn and engage with on the whole.
No systemd, no convoluted outcomes, but still all the choice. Want to add to the core OS and add a desktop environment to it? Go for it - just install xorg and the DE of your choice and you're done. Talking of installing, you can install binaries (eg pkg install mate-desktop), you can build from source... or you can use Ports. Think Arch AUR on steriords and done right. The Ports Tree is a map of the software you 'could' install, saved to your machine in /usr/ports. You can search through it in a few seconds using the terminal and then simply cd to the appropriate directory and install the latest version. For example:
cd /usr/ports
portsnap fetch (grab the latest copy)
find openvpn (output : /usr/ports/security/openvpn-2.4.6)
cd /usr/ports/security/openvpn-2.4.6
make
make install
make clean
Done. Yes it's a bit more typing than 'apt install xyz', but it opens up a simply astonishing range of extra software ready to be built and customised to your exact machine and environment. You can always just stick with binaries, but ports are very useful at times. At first you can find it a little overwhelming - installing xorg (though in the end it's as simple as 'pkg install xorg' and it will configure itself), adding a DE and setting things up how you like. With the excellent free Handbook available on each BSD OS website on hand, you quickly find answers (usually just a different way to doing things than Linux), and rapidly realise how 'common sense' and easy things actually are. For example, most of your config/options will go into a single file (/etc/rc.local) and are as simple as - for example - 'gnome_enable="YES"' and so on. Once you're over the initial hurdle of 'twigging' how it works it's a very easy OS to work with and everything is logically laid out and very well (exceptionally well, in the case of OpenBSD) documented.
It still astonishes me how flexible and powerful BSD is, even compared to Linux. DragonFly BSD with Hammer2 (file system) is almost comically fast even doing something as simple as opening a file manager. You thought Windows/Debian/whatever was fast already? LOL... No. It's even more amazing when you consider DragonFly BSD was basically mostly hand written by one single guy, who still runs the project. He also single-handedly wrote the Hammer and Hammer2 file systems, which are essentially multi-threaded modernised ZFS type systems.
Finally there's licensing. The BSD licence is much more permissive and far less ideological than the GPL. It basically says 'Do what you want with this code. Use it commercially if you like. Just don't sue us and make sure you leave the copyright notice in for the University of Berkeley'. Simple. Hence it's very popular with companies like Netflix, who run their entire CDN on FreeBSD and have contributed a monumental amount of code and tweaks to the networking, IO, and other stacks in the kernel. FreeBSD can now serve video at around 100Gbps without flinching and has a very optimised network stack. Not to mention most ISPs use it for similar reasons.
TL,DR: BSD is pretty awesome once you start to play with it and learn just how powerful and flexible it is. You can do anything with it, even easier than Linux, and it's hella secure and fast to boot. Win. You start with a couple of hundred MB ISO, and before you realise it you've learnt a ton about how to actually administrate your system and are able to install various DEs, configure hardware and drivers using config files, and know exactly what's going on under the hood. You also have a custom made system that's perfect for you and your needs. I like it a lot... but Linux is still cool.
They'll pull my MBP, iPhone X, iPad etc from my cold, dead hands. It's hardly BSD these days though (but still my favourite OS).
I'm looking forward to reading it.
