What are the differences between Counter Strike 1.6 and CS:Source?

These two games use different engines. I was going to write some differences that I remember, but then I found a set of videos on YouTube that compare a lot of things between CS 1.6 and CS:S. Here's the final result of these videos:

A short explanation of every point:

• Grenade Sizes: they are bigger in 1.6. So it's somewhat easier to see them and get a chance to evade them.
• Grenade Physics: grenades in 1.6 fly where you throw them. They are predictable and if they bounce from the wall you do know where will they land (more or less).
• Player-World Scale: CS:S looks smaller. On dust2 you can just jump on the higher part that is on B, the boxes there are headshot-boxes and not a stack of 2 anymore. And these low boxes give an advantage to the Terrorists.
• Movement: pretty much the same, but in CS:S if you move in one direction and then you immediately move to the other one there is some lag.
• No Barrels & Junk: CS:S has moveable physics-based objects, CS doesn't.
• Weapons: more powerful M4 in CS:S and less effective AK47. More info in the video.

• Wall Spamming: shooting through the walls. In CS:S you can only shoot through tiny walls and/or doors, in 1.6 you can shoot almost through everything(if you played fy_iceworld in 1.5 of 1.6 you should know that people can easily get a kill through 2 huge iceblocks with an AWP at the very beginning of a round).

• Smokes: smoke from an SG is thicker and it appears not immediately. You can't see through it and 1 SG is enough to obscure the visibility. In 1.6 you needed 2 SGs to make the smoke solid.

• Grenades (HE): the difference is that in CS:S the grenades do not hurt people standing behind the wall. In 1.6 if you would throw a grenade to the wall and an enemy would stand just behind the wall you could possible kill him.
• Graphics: CS:S uses Source engine, CS GoldSrc.

Unfortunately, the video is no longer available, but the above information is still correct.

The main difference is the way you use weapons. For example weapons shoot differently, they tend to spray a bit more, causing a loss of accuracy. Grenades are not thrown the same way (there is a different gravity). Maps have been remodeled so there's some box or barrel added to add some more safe spots. There isn't much more mechanics to the game, it still is the same game.