Hi, i just want to know is there any level, boss fight or stage that's only playable on certain difficulty? If so, can somebody list them for me?

I already done plenty of google-ing, but most of the articles are incomplete.

From the new Painkiller HD walkthrough on Youtube, seems like certain level are missing on normal difficulty (Prison, Asylum). I assume this is same with the original Painkiller since the HD is basically the same game with an improved graphic. However, i'm not really sure.

I've never played on Daydream, but I know that Prison (Chapter II level 1) is available in Nightmare and Trauma, and Forest (Chapter IV level 4) is in Trauma only. Can't recall how the maps worked in Battle out of Hell, but I do remeber there being a secret bonus level after completing the game on Trauma.

Because nothing says replay value by setting a bar of required skill in order to be able to fully appreciate (i.e. access) all of the game's content.

Sorry, but this really irritates me. I am a fairly decent gamer, but the prospect of going through some of these levels at higher difficulties is not encouraging. Regardless, I still paid the same amount for the same amount of game content, but because of my shortcomings, I am effectively unable to play and enjoy these levels short of engaging in tedious hitting of the load button and memorizing the layout.

This is just common practice in video game. Certain developer prefer to make more extra content accessible after beating the game in fully or on hardest difficulty. This is certainly throwback to the old school unlocking style as People Can Fly developed the game with old school FPS mindset. You can find similar example in virtually every video game out there. For latest example, check out Resident Evil 6.

Beside, among all of old school FPS i found Painkiller is definitely among the easiest even or hardest difficulty.
- Enemy died after taking 1 or 2 hit even if they come in pack.
- Your melee is the most powerful weapon.
- Clear waypoint indicator so you won't lost your track and next checkpoint 90% of the time.
- Health bar for boss fight
- Most importantly your objective in this game is just kill all those creatures using all of your arsenal of weapon, nothing complex.

For example, Serious Sam without a doubt is far more punishing than Painkiller even on normal difficulty.

Custom does not make this less annoying in any context. I can excuse games having unlockable extras, including special levels, but having levels of the main campaign be designated as off limits to lower levels of difficulty is something that we rightfully abandoned long ago after recognizing it for something that is generally employed to skimp on having to create extra content while still providing a way to pad the game. Even if Painkiller does not do so for this purpose, the fact remains that this is an antiquated design choice that is abrasive due to how out of practice it is.

Also, while Painkiller might not be as punishing as other games on higher difficulties, for me, it was still fairly challenging on normal in that I came close to dying and died at a somewhat lower but respectable frequency, while harder modes made me want to tear out my hair. Maybe it was the fact that I hadn't memorized each and every level as I explored it or I wasn't as quick-fingered as I should have been, but the end result was that I was effectively locked out of portions of the main campaign, something that by definition should be accessible from the word "go".

It is accessible from the word "Go," but it requires a bit of work to get there. Just like every other FPS that there has ever been. I understand it is frustrating, but then so is every FPS game at times. To end this thread, can you please provide an example of a famous FPS that does not force you to progress through the game before you see all the levels?

You seem to be misunderstanding me. I am miffed that the game prevents you from playing certain core levels if you are on a lower difficulty. These are not secret levels which are added on to the main campaign that require some impressive feats of discovery and reflexes in order to access, but regular levels that the game forces you to skip because it has decided you don't get to play them at lower difficulty.

To illustrate my point, imagine if Doom, instead of merely having levels that could be accessed via secret entrances, skipped from E1M3 to E1M5 if you were on "Hurt Me Plenty". The level in question is not secret in nature, and is positioned such that it is the next in line of the levels made available in the campaign. When playing the core game, my access to the core game should not be arbitrarily determined by the game designers blacking out some levels for lower difficulty settings. It's akin to reading a book and being told to skip a chapter because the author has decided that people below a certain reading level are unfit to experience some of the chapters therein.

I just finished painkiller on Trauma mode, I didn't mind playing through the game again as it was so much fun each time. I died several times trying to complete the Forest level, it is quite a challenge!

Then again, many games have progressing difficulty, so you'd need to improve in order to complete the end levels. I agree, having levels restricted to a certain difficulty is a cheesy way to add more playtime, but in case of Painkiller I didn't mind. Whereas the core gameplay is very similar, each level is so different from each other that I never got bored with them. I think I've completed "Heaven's got a Hitman" eight times by now, mostly on Trauma or Nightmare, and I know I'll do it again, if only to admire the level design and the sheer number of details they have.