The game is one of the best FPS games on steam. Why? Well because the developers got a lot of things right.

Unlike most FPS games you don't play a generic solider, but you play as one of 9 classes that are all unique. That alone creates a lot of playstyles and variety. Another thing that the game does well it that there are no upgrades for your weapons but there are only sidegrades. That means that the stock loadout is good no matter if you just started or have many hours. The game also does microtransactions very well, because they are mostly cosmetic items (the exception is the tour of duty ticket and the competitive pass).
The game has many game modes. You have casual where all the bots are a big problem, because it is hard to kick them and are simple a nuisance (This is a problem, because the game hasn't been getting the attention it needs from Valve). Competitive where you wait a long time to find a game.. And when you find a match it's most likely off balance. Mann vs. Machine that is a cooperative game mode and has 2 sub modes. Mann up where you need to buy the tickets and boot camp that you can play for free. The main reason people play Mann up is because they want good loot and by that I mean everybody thinks they will get a golden frying pan (the australium weapon that has the highest value) if they play enough. And of course you have community servers that can be fun if you like mods. Overall if you can look past the bots the game is still fun after all these years. The most important part of the game is that you don't play it to seriously, because there are servers that people just want to do the conga and maps where people simple don't want the round to end (hightower and 2fort mostly).