TF2 should be the greatest multiplayer FPS ever made. Ten years later, it's still the best at what it does in so many respects it's a little disappointing that no one else has beaten it yet, and I attribute that to the stupid little road blocks Valve has put in the way of the game actually being the greatest.
It's easy to pick up. New players might not really understand exactly what's going on on the screen at all times, but Valve's pubs are so low-stakes that a player who has never even played video games before will have time to just fuck around and build a sentry, a teleporter, and a dispenser on last, while still having the opportunity to watch other players performing basic functions like rocket jumping out of spawn. If they discover their w key they can wander over to the front lines and watch players all over the skill spectrum duke it out, all with no pressure outside of their own desire to play well. You can just play as hard or as easy as you like, learning the basics of gaming and being allowed to advance at your own pace. You don't need to read tutorials, you don't need to ask for help; until you really start trying to get competitive the game explains itself stunningly well by just letting you fuck around.
Valve's road blocks here are numerous, though. Random crits, random spread, long respawn times, joke weapons like the base jumper
and classes like the pyro are all directly opposed to the perfect learning environment that I feel pubs should, and easily could be.
It's satisfying to play. Playing the game just feels good. As long as you make it out of spawn, doing things in this game feels great. Hitting headshots, landing directs, watching your sentry melt idiots, backstabs, ubers, they all look cool, make really nice sounds, and make good things happen for you. These things will happen as you play, no matter how bad you are. This is why the game is also satisfying to get better at, because these things happen more often.
Random crits are also detrimental here, rewarding players for stupid shit, but the bigger issue is the lack of an easy transition into competitive play. As good as it feels to get better, there is a very real skill ceiling that exists in pubs because there is no skill matching, and you get to a point where you can melt new players all day and only occasionally meet other players that actually know what's happening. It's legitimately a tragedy that pubs do such a good job at teaching players the basics, only for the game to completely fail to bridge them into the next level of play.
It's easy to watch. Pubs and highlander are difficult, but that has less to do with the game and more with the sheer number of fights that are happening at the same time. Individual fights are both simple to follow but surprisingly complex to analyze, and that's what make 6s so good, in that there are almost never more than 2 fights happening at once. I know I'm in the minority on this, but at the top level I even find stalemates fun to watch, as most teams (at least in NA) spend the stalemate poking each other and trying to find ways around the opponent's flank. Compared to Overwatch or most MOBAs, where even watching small fights can be made confusing by all the flashing lights and contrasting noises, not to mention the requirement that you know what every single one of the 30-something heroes do, TF2 is a goddamn breeze to watch. I remember watching i55 at my grandparents house when I first started playing TF2 competitively, answering questions like "why is that guy glowing" and "what does frag mean?" Not 10 minutes after we started watching, they knew the basics of when a team was doing well, just by virtue of the ubercharge bar and the cap point boxes. Shit's easy.
I could go into all sorts of teamplay shit too, but, uh, that's why I'm here. TF2 is fucking incredible. Unmatched. There's nothing like it, and I want to do what I can to promote and experience the beautiful game design showcased here.