This is where IO Interactive mastered the Hitman formula. Each level is only loosely linked to the last (for the most part), each one stands out as unique and memorable, and they are all fairly large, open-ended and "alive" with NPCs going about their business. You have more options than ever before, many more creative ways of dealing with your targets, and the overarching story takes a back seat, mainly only playing out in cutscenes between missions, which is how Hitman should be. It's highly imaginative and vastly improved over any of the Hitman games that came before it, even though the core mechanics work pretty much the same way.
One main criticism I can think of is that the game engine was never particularly refined, and has a lot of oddities. These days it's showing its age, not just visually (which goes without saying) but mechanically too, and it can sometimes feel clunky or act strangely. The game encourages precise strategies, and repetition to master those strategies, which is important to the way Hitman works... But you may find that some of the awkwardness of the game's internal "rules" under the hood can occasionally stand in the way of your precision. The notoriety system isn't very good either, but I would be the 50,000th person to complain about that, and I can't be bothered.
Anyway, it's not a perfect game, but it was the perfect Hitman game for a very long time. That feeling after pulling off a successful, silent, and creative assassination; then strolling away without anyone the wiser... There's nothing quite like that feeling, and this game was the king of that for 10 whole years. Only 2016's Hitman does it better, and no other game series has come close. I'd say this game was and still is a must-play.