Here are the differences that I've observed (some may be already mentioned): I no longer have CS 1.6 but I still got Condition Zero, which is, in my opinion, plays identical to 1.6, except for the overpowered spread weapons (HE, Shotguns, Machine Gun).
Unfortunately, the video is no longer available, but the above information is still correct. Graphics: CS:S uses Source engine, CS GoldSrc.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. Grenades (HE): the difference is that in CS:S the grenades do not hurt people standing behind the wall.In 1.6 you needed 2 SGs to make the smoke solid. You can't see through it and 1 SG is enough to obscure the visibility. Smokes: smoke from an SG is thicker and it appears not immediately. 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). Wall Spamming: shooting through the walls. Weapons: more powerful M4 in CS:S and less effective AK47.No Barrels & Junk: CS:S has moveable physics-based objects, CS doesn't.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.And these low boxes give an advantage to the Terrorists. 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. Player-World Scale: CS:S looks smaller.They are predictable and if they bounce from the wall you do know where will they land (more or less). Grenade Physics: grenades in 1.6 fly where you throw them.So it's somewhat easier to see them and get a chance to evade them. Grenade Sizes: they are bigger in 1.6.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.