If you want to bring up mechanics...
1.6 you can shoot through thick walls to kill people with a pistol if you know where likely hiding spots are on the other side. Wallbanging.
1.6 has a shield for CT which is bulletproof even against sniper rifles despite the fact that guns can kill through concrete.
1.6 when you throw a grenade it floats through the air like a beer can in slow motion.
1.6 sound is not directional, someone walks up to you, you hear footsteps but unless you already know the directions they might be coming from you can't tell where from.
1.6 is heavily populated by players who tried the new game, were upset by the differences and chose instead to stay with the old game they knew rather than migrate.
The graphics are dated, model movement is snappy not smooth. There's a lot of things against it. But when you get right down to it the game is the same. Team A, Team B, kill each other/complete a mission objective.
But play enough of either and you'll grow to like it because your game awareness and skills will improve for that specific version and naturally you'll do better. Just don't expect skills to transfer too well to the other.