Numbers: Things hit harde/faster, also you die quicker.

Mechanics: Addition of other effects that can often synergise and require completely different priorities so you don't get FUBAR'd three ways from Sunday.

Take the Left 4 Dead games. In L4D1 you had various spots most of the Horde would get stuck trying to reach you so you could turtle down with twin pistols that had infinite ammo and slowly chip things down. For any Special that spawns you could still chip them and then swap to your primary weapon if really needed for that extra DPS burst, especially for Tanks.

L4D2 make this both harder and all but impossible by not only fixing many spots that you weren't really meant to get to in the first place, like the rock near the finale house in Death Toll, but added in new Specials that either specifically caused area denial of camping spots or raised the importance of positioning and situational awareness signiticantly beyond just fighting Tanks. For the former you have Spitters that covered the area in acid, for the latter Chargers that could very quickly close the distance on Survivors and scatter them in addition to carrying a primary target away/off a cliff.

Mass Effect 3 did this as well with updates to its multiplayer system after players would frequently pick Firebase White vs Geth since the former had great camping spots pre-Retaliation nerfs and Geth had no mobs which had auto 1 shot sync-kills. While the nerf didn't change the latter it did add drones to the Geth that would carpet bomb an area with timed explosives as well as adding in several new approaches to the former camping spots, both mechanical changes in regards to this topic.

So going from L4D1 to L4D2 the change in numbers and mechanics were:

Numbers: Same as in L4D1, harder modes meant Survivors took more damage, zombies needed more damage to die, no real difference between the two games.

Mechanics: Daytime maps which means wandering Witches, sometimes a good thing because it made getting past them simpler but also the opposite if they patrolled an area like a hallway between sets of stairs on The Parish map which required precise timing and a lot of waiting to not startle her. Weather. Heavy Rain in particular focuses on this when the Survivors start the 5 map campaign with comments made about a storm coming. By the time you make it to the halfway point and pick up the gas cans there is both rain and large puddles which slow movement. On the return journey all 3 subsequent maps, of which the 3rd is the finale holdout that includes Horde waves, Specials, and Tanks, there would be random squalls that impeded vision, hearing, and consequently your ability to shoot things. While you could temporarily hunker down somewhere for a few minutes and wait for this to pass, it wouldn't stop anything attacking you. So any lingering zombies would still charge after you, Specials would still spawn and make things even more difficult depending on which ones spawns, like Spitters for the reason mentioned above regarding area denial, Witches which were now back in their L4D1 sitting/crying poses and the Sugar Mill specifically spawns multiple Witches making them a guaranteed encounter to deal with in this campaign, and most importantly and Tanks you were unfortunate enough to run into.

Many of these new mechanics would also synergise to go back to that point. If a Hunter had you pinned then a Spitter can make things even worse by not only spitting on you to increase the damage you take, but damage any other Survivors trying to make it to you attempting to either knock the Hunter off if ranged weapons are limited or help you Recover if you took enough damage to collapse, aka area denial.

Not only the above, but also "story" changes where Survivors would start off with the bare minimum of weapons. No Mercy, the first L4D map gives you a primary ranged weapon from the get go, be that a shotgun or uzi. You don't get that in L4D2 Dead Center until you get down the elevator after already 2/3rds or 3/4ths through the first map, and when you do get your first set of primary weapon spawns outside the elevator the room is on fire, smoke is everywhere, and opening the doors spawns a Horde that rushes you which may also include some Special spawns with it. Until that point you start with either getting to pick a second pistol or melee weapons, which are at least extremely effective vs normal zombies although have mixed success vs Specials for different reasons.