After 6-7 years, I decided to buy Witcher and I bought it a few minutes ago. :))
Okay. My question is just about replay value of this game: difficulties, modes, new game+, secrets, and other replay factors.
More than one playthrough OR after one playthrough I should say goodbye to Witcher? Please give me some details.

For the next 7 years (2021), I planned to buy The Witcher 2!... ROFL...

If you like the game at all, you will probably want to play it at least 3 times.
Supporting each faction or remaining neutral.
You can't see all the content from any one play through.

"3+ playthroughs" sounds good.

Also, the mini adventures are fun and remind me of The Last Wish short stories. Although, I say play through the game first to get used to the way the game works and play the adventures later.

Has Witcher New Game+ mode or export character feature (export character like Sacred)?

While I do own Sacred, I haven't been able to get it to work, so I am afraid I don't quite understand what you mean.

Really? That's simple. Just export your character in Sacred/Sacred Underworld/Sacred Gold and then when you start a new game, you can import your exported characters and play with them.

No, nothing like it. Your Geralt starts every game with amnesia and a rusty sword.

And difficulties? Easy, Normal, Hard, Very Hard, and etc.?

Multiple difficulties, never truly an easy game. I preferred playing from low isometric (there are three camera options), and when I try and play Sacred it encounters enough frame rate issues that I just can't play it (supposedly having both Windows 7 and an Nvidia card will do that).

Three difficulty levels: Easy, Normal, Hard. Even Hard is easy once you get used to combat. The Prologue does a good job of introducing you to the styles and rhythm of combat. Combat actions have to be timed. The game will frustrate button mashers.

Average time for one playthrough? Over 20 hours?

Lots more, it's not a fast-paced game, the main quest is intricate, there are a lot of side quests, and I slow it down even further by being a leisurely player. GameLengths says 45 hours; I take more like 60.

Some find it too slow-paced, especially the first act.

Then The Witcher will be one of my favorite games. Nice.

A lot more. As a small comparison, I've already put in 10+ hours, and have only started on Chapter 2.

One feature that increases replay value is that some choices you make will have unforeseen consequences down the line. The paths here are mutually exclusive, and due to delayed consequences you can't just reload, pick a different choice and see the consequence right away. Off the top of my head, a choice you make in the prologue will have a consequence in chapter 1, and a choice you make in chapter 1 will have a consequence in chapter 2.