Its hard to put into words how bad source was in comparison to 1.6. Source got better, but it was never even close to 1.6 in terms of gameplay. Just about every single thing that was bad about source was a change to 1.6. Let me list the ways, from memory.

1. The hitboxes were busted. They were consistently not where they were supposed to be - generally behind where you were aiming. Something similar exists in CS:GO but to a much less extent.
2. The recoil was compressed making the game easier.
3. The movement was shit. Not only was bunnyhopping INSANE, whereas it was not that bad in 1.6, but just running felt shitty. You always slid and your jumps weren't consistent because of the momentum system. Similar issues exist in CS:GO but aren't nearly as bad as they were in source. It also made walking neigh impossible because you lost ALL SPEED gained. Who cares about sound right?
4. The models, and their hitboxes were HUGE. The maps were largely the same(plus physic objects, which i'll talk about later) so it sometimes you'd shoot a guy in the chest and get a headshot. It felt stupid. It also made certain guns(awp, AK, deagle) more OP then they already were in 1.6. The difference was insane.
5. Physic objects broke the map. Most of the time it wasn't a big deal, who cares about the barrels on long A really? But when they were in the site they were GAME. BREAKING. You could roll them over the bomb and it could literally make it impossible to defuse. Luckily you could shoot them off. Pray your buddy didn't die trying to defuse because the defuse kit would BLOCK YOUR DEFUSE FOREVER. IDK if they fix that but that was horrible.
6. Crouch spamming was stupid. You could see them and they couldn't see you. It was so dumb. They fixed it in GO by adding stamina but even then it still has an issue. 1.6 had its only issue with crouch, known as russian walking, but it was incredibly hard to really abuse vs just spamming cntrl like the issue was in source.
7. They broke the money system for a short time period. For a short time there was a real economy to the game, where the more you bought a gun the more it cost due to the lack of supply. That made ammo(which was purchaseable previously) almost impossible to buy so they removed that - removin gsome of the depth of 1.6. But what really made it blow was that you couldn't buy an AK, M4 AWP without 16K. Even then you couldn't buy ammo. You could buy a pp7 though, that cost 5 dollars. Enjoy your no kills.
8. Personal thing - the fact you could channel Dan Marino and throw a god damn grenade over fucking buildings was supid. Throwing a grenade from T spawn to flash B on dust2 was stupid. The fact you could effectively smoke connector from FUCKING T SPAWN WAS SO DUMB WHAT THE HELL. This is still an issue in CS:GO but its not as bad. I'd rather it not exist but what do you want really?

So just imagine combing #1 and #4 and you have a game where you could completely miss the person and get a kill. Thats a major reason why most of us 1.6 players thought it was stupid.