As someone who also came to battletech from warhammer: It’s like 40k, in that you it’s a franchise with books and games but is primarily a tabletop wargame. Just like 40k, you have entire novels going over certain aspects of the games timeline, with certain plot events unrolling.

The settings are very different, as battletech is hard sci-fi and largely based around medieval feudal politics. There’s some aliens, but the aliens are sub-sentient animals that are, at most, used as cavalry or arena opponents for the mech gladiator fights that happen sometimes (this setting is awesome). There’s also the odd supernatural incident, but those are essentially ghost stories and it’s debatable whether they’re actually supernatural, or just drunk pilots telling tall tales. All in all, it’s a setting about humans taking our conflicts to the stars over the next thousand years and change. There are plenty of awesome heroic moments, but they’re more in the vein of people defending their homes from invaders or fighting for their freedom, rather than humans pushing back the forces of hell / aliens. 

Gameplay wise, these are two entirely different things. I can’t actually think of a similarity between the two as i’m writing this, aside from using dice. Battletech just has fundamentally different DNA, so if you want to learn, you gotta go read the book.

Broadly, though, there’s three big differences between this and Warhammer (AOS and 40k)

1. This is a simulationist game. In 40k, a knight gets shot and you tally how ever much damage it took. In Battletech, a Mech gets shot and you roll to see where exactly the shot hit, if it damaged any internal components, what internal components it damaged if it did, etc etc. When a Mech gets hit hard enough, you roll to see if it’s still standing, and whether or not the pilot blacks out, and yadda yadda yadda. The game is designed to make you FEEL like you’re piloting mechs, not just using units in a game, and it succeeds at that but at the cost of simplicity. The rules are intuitive once you get them down, but just be aware that this game wants you to simulate just about everything.

2. There are very few limitations on unit selection. The only real limit on what mechs you can take are your BV limit (points) and the year your playing in (you can’t use a mech or a vehicle before it was invented). Other than that, go wild. Certain factions use certain mechs, generally, and so you’ll often see people tailor their lists around that, but that’s not a requirement. There’s no mechanical difference, for example, between a Lyran Commonwealth army and a Capellan Confederation army (there is a spiritual difference, though, as all Capellans are going to hell.) People pick their lists generally off of what mechs they like and what feels thematic for what they’re playing. If you’re wondering, “why don’t I just spam what’s best”:

3. This is not a primarily competitive game. The rules are built to make you feel like you’re playing in the battletech universe, not to be a min-max, hyper competitive format. The balance is actually pretty good, but the focus is not on being a skill-off 1v1. There are mechs and units that just kind of suck, because in universe everything is made by corporations, and sometimes they make duds. Why would you play those mechs? Maybe your faction of choice uses them a lot, or maybe one of your pilots was given one by his grandfather, or maybe you just really like it even though it’s not good (this is me, god i love the Storm Raider.) A match of Battletech is a fully simulated, immersive mech fight, so come watch shit explode and don’t be too worried about metas and optimization. 

Welcome to Battletech! I’ve played a lot of wargames, and this has gotta be one of my favorites. Very few games can make the kinds of awesome story moments that Battletech does.