This made it virtually impossible to level up.
Even if I write a really high-quality post, maybe 100 people really like it but only 2 of them have spendable merit because they used it already.
I don't understand how this system is supposed to help anyone level up?
Instead, it just makes sure that the people who already put in the work previously, will be rewarded for it.
It's virtually impossible to gather 250/ 500/ 1000 merit scores if you don't have the rank yet.
Really? I started on 250 Merit as a Sr Member so I need 250 to rank up to Hero. Since 24th January I've got 147 of those so only another 103 to get. It takes 8.5 months to get the activity required, so this isn't going to slow people down at all, let alone make it impossible.
What magic are you using to get so many merits even for short posts?

I have created a popular thread about privacy coins, but received only few merits so far. Maybe to get some merits one also has to be very lucky

Although I agree with the sentiment of your comment, and indeed made the same observation just above you, I also took the liberty of looking at the post about privacy coins you referred to. The problem I see there is that it's not original and does not contribute something new. There are already several threads looking at the universe of privacy coins, some of them with much more details. Taken on it's own, the post is of high enough quality and thus deserving merits (which it got). But I would not give it a merit myself, again, because the topic is being discussed on many other threads, and personally, I would prefer to see the other discussions advance.