1. Sounds on Different Surfaces
There are lots of different surface materials in CS:GO, and each of them has its own unique sound. An experienced player can easily tell the difference between metal, grass or wood. The sounds when objects fall onto these surfaces are different too. Try to throw the bomb on tile, a wood floor and a chair. You'll hear distinctly different sounds with the tile and wood floor, and the bomb landing on a chair makes no sound at all.
Your opponents cannot hear all the sounds that you can. Here's a simple example: you can easily hear the sound of pulling a nade's pin, but your opponent won't be able to hear it even if they are just around the corner. However, the sounds of a nade bouncing off a wall or of throwing one can be heard by everyone.
3. Shooting & Modes
While switching the firing modes of the Glock-18 or FAMAS, players that are close to you will hear a specific sound. Also, the sound of reloading can easily be heard, too, so it's better not to reload your gun after two shots every time.
4. The Fastest Way to Plant the Bomb
This is the most obvious and simple advice that many CS:GO players forget. In order to plant the bomb as fast as possible, you can just press "E" when you are at the bomb site. Your character will pull out the bomb and start planting it. Also, if you release the button, your weapon will immediately be back in your hands, ready to frag more CTs. Isn't that convenient?
5. New Decoy Tricking
After the release of Panorama UI, the decoy grenade is now shown differently on the radar. Your opponents used to always see a red dot on the map, but since the Panaroma UI update, the player and their teammates now see their own decoys, while their opponents don't have it marked on their map even if they are looking straight at it.