I've come to enjoy KF2 much more than the old KF1.
Where as KF1 looked more gritty and scary, KF2 looks a lot more polished and has better graphics; of course that's due to updated engine software though. But you can still make the maps in the game look hellish after turning every zed in a 5 mile radius into paste, and it gets better once you figure out how to edit the gore settings in the KFGame file. Some still want that old move theme slasher feeling, which I don't mind, but I get more out of seeing zed corpses and pieces lying on the ground with the walls and ceiling painting with their blood in vivid detail.
As for gameplay...I'll be honest. I probably have way more hours with KF2 than 1. For crying out loud, the second game has a shove function that the first game lacked; and keep in mind it came out around the same time L4D2 did. If you were grabbed by a clot and didnt kill him quick, you were essentially screwed. Not to mention the only way to run quicker was to hold your knife out. In my opinion that doesn't make much sense, since you're still carrying the same amount of weight no matter what you have out. Plus, it kinda makes you a bit vulnerable since you have to change weapons of you need to shoot something; not to mention to even use your flashlight, you have to have your pistol or whatever weapon its attached to out. Not very fun in dark areas where your best weapon (if it doesn't have a flashlight) would be more useful. The second game fixed those by finally adding the shove function so those clots can piss off, the ability to sprint being tied to a key, and a body flashlight.
Then, we have the grinding. KF1's grinding system was terrible. You had to do specific condition to level up, or otherwise you couldn't. Welding doors as a support consistently became a real chore, and the commando had the infamous issue of having to kill X stalkers (which we all know was hard when your teammates wind up killing them before you do). You'd be likely stuck at a certain level for a few weeks of things didn't go your way. Sure, getting a higher tiered weapon made things kinda worth it, but that's questionable. In KF2, it was much more streamlined where all you had to do was play a perk for a week to reach 25, and along the way you could pick certain perk skills that could better suit your playstyle. Oh, and the best part? Doing certain objectives or killing certain zeds counted as bonus XP. In short, just playing the game guaranteed you leveling up, and you could reach as high as level 10 within 2 days.
I would say perk skills replaced the notion of starting with a higher tiered weapon when you spawn in. Getting a grenade launcher when you spawn as a demo is nice and saves dosh, but zed time skills can be a real game changer for almost any perk.
Now, as far as the zeds, stalkers, crawlers, and sirens are still as annoying as ever. Clots are a bit better and the three variants make it a little interesting. Gorefiends were extremely annoying until you could finally interrupt their blender attack. Can't say much about the bloat, but the husk is effectively the bane of everyone's existence if you don't know where he is. As for the big ones, sure, the roars give them away when they spawn; but as far as exact location, they're still ambiguous (usually right around the corner...). They're also more dangerous than in the first game since stun locking them is no longer viable with a zerk, not that it was rewarding in the first place, and they make it harder for the player to hit their head once they start to rage. Fleshpounds will always rage no matter what you do, and the fact they can now spawn enraged makes them even more dangerous.
However, one good thing to note is that all zeds can activately attack each other if you trick them into crossing their firing line, especially with husks and bloats. You can't say it's not amusing to watch two or 3 husks trying to roast each other and one ends up suiciding to kill them all.
Oh, and about the whole thing with 'fire on big zeds'....
Large zeds are big on fire resistance. The only weapons that could effectively take care of them were the microwave gun and the husk cannon, but they got nerfed and aren't as useful as they used to be. While setting one on fire does throw him off your team's back for a few seconds, it does ruin the shots of those trying to end him. That's why firebugs usually don't deal with big zeds. Besides, they get more out of killing everything that's NOT a big zed.
Oh, and before I forget, there's the maps with this game. Just about all of them were designed with some level of run and gun play. KF1 was filled with corridor maps that made it easy to find a decent spot and camp, but in this game, that strategy is mitigated with the fact zeds can come from almost anywhere like vents or sewer pipes. If you need to abandon your hold, it's best to do so before you get overwhelmed. Maps like Nuked and outpost are pretty good examples. I guess TW dove into the original concept with the holdout maps like Krampus Lair, Descent, and Powercore, but there's still some room for you to run around.
In short, KF2 expanded on things that KF1 lacked or could've had. KF1 is still a decent game, but it only went so far with how it was developed and felt like some things were missing, frustrating when they didnt need to be, or could've been done better. And that's what I see TW did with their sequel.