The cinematic trailer showed one hit snipers did it not !
The cinematics also showed a rocket launcher and a backflip blablabla to dodge it. Which both are NOT in the game, but it's just cool for a cinematic.
Besides snipers are confirmed to not be OHKO. The video you were referring to is probably the latest dev diary. It's also not a sniper, it's an Highpowered AR with a scope. The guy who got shot toook probably some damage before.
Ontopic:
There's a "stealth class", the operative. He can disguise himself when he steals the outfit of a member of the other team. Only other operatives can spot disguised people. They are also able to hack certain objectives. Combine this class with a light body type (which gives you faster movement and more options with the SMART system) and you can be pretty stealthy. In addition, operatives can do backstabs which are OHKO's (if I remember correctly)
As for the mindless shooting and ducking.. This game is Objective based. Every kill you do, helps the team in pushing forward and completing objectives. The defending team wins if they can prevent the attacking team from completing the main objectives (F.e. Blow something up) The attacking teams wins only when they completed all main objectives. If your team consists of lonewolfs only, who are focused on killing, then you'll lose without a doubt. However, there are four classes which all serve a purpose. Like they said earlier, read the compendium, it contains a lot of information. (It's not an official topic though)