Take the Spy. He’s always been able to disguise himself as any enemy and moonwalk casually into their base. But now he can cloak too, vanishing entirely for that whole awkward, highly suspicious infiltration period. Once he’s in, he doesn’t have the nailgun, shotgun, gas grenades and tranquillizer darts of old. A flick-knife and a small-caliber pistol are all he needs to ply his dark trade.
It’s now so easy to slip behind enemy lines that playing a Spy becomes not about whether you can get a kill, but who needs killing most. The lone ranger has become a team player: everyone relies on him to take out the most dangerous player in heavy enemy fortifications. And when he strikes - with a sudden reveal and a sharp flick of his blade - he can usually vanish again before he’s julienned by enemy fire. Only to find another dark corner and reappear as someone else. It’s subterfuge taken to the logical extreme; what doesn’t serve it has been scrapped.
The Spy’s bound to be an early favorite - apart from anything he just looks so goddamn suave - but we actually had much more fun as the Medic. Their healing ray was one of the main reasons Valve had to scrap the realistic art style: they prototyped this insane device and it was just too much fun to leave out. It locks onto a friendly and pumps them full of health points, beyond their normal maximum, charging up all the while. By the time you’ve buffed most of your team, you’ll be at full charge and ready to go. This makes you and the guy you’re healing impervious to all damage for around ten seconds, sending both of you on a bloodthirsty rampage into enemy territory.
By this point you’ve already formed a close relationship with your preferred healee: they’ve been able to tank absurd amounts of damage while you stood behind them pumping them with plus-signs, and they’ve fearlessly put their body between you and danger. But now you’re true brothers in arms: after all your careful cooperation and strategic retreats, the two of you are suddenly the most devastating thing on the battlefield. When the magic finally expires, many kills later, you dash to a safe spot, pant, and heal up for another go. It’s another great example of that functional exaggeration: you’re so good at healing, as a Medic, that it’s virtually all you do.