For the other points you make, I'll do my best to answer your concerns:

1. There is no indication if there are some new posts inside some of the subforums/groups. We have to open them to take a look.
The forums are highlighted with a different colour icon if there are new posts since you last visited. I do agree that the colouring is slight and vague and doesn't really make it obvious there are new posts. We will update the icons to better display forums with and without unread posts. Thank you for pointing this out.

2. Even after opening some subforum/group icons representing unread posts (open envelope) do not differ much from read ones (closed envelope). The color indicates actuality of the posts, but it would be much better to draw attention to unread posts.
If I understand again, this is an issue of unclear icons beside the posts. We can fix that! Also, unread posts have a button to take you directly to the first unread post, which might help.

3. I have just opened two browsers on the same computer. In one it is indicated that I have read post (and that is true), while on the other "the envelope is opened".
From your comment further down, you are not logged in, so there is no way for the browsers to know what you have or have not read. Once you login, this issue should be moot. Let me know if I missed the point.

4. What is "advanced" in writing reply using "Go Advanced" button. The interface is really Spartan.
Yes, I agree with you on this. The reply form needs some attention. I'll get to that right away and see what can be done to make the advanced form more useful.

5. I have problem to login on the smartphone, and also many images are missing (just placeholders with "imag" text appears). I'm using Opera Mobile. The site looks completely different than on a desktop, but ... Since I cannot login I cannot try anything.
You can switch to the full theme if you prefer that on the smart phone, down at the bottom there is a link for "Full Site". When you arrive on the site with your smartphone, can you tell me which images are broken? I'm not seeing what you are seeing. I've also tried several times to login and had no issues from a smart phone. Which is your phone make and model, and do you have a different browser you can try from the smart phone.

We'll keep working to improve the "default" icons and features to make the forums better than they ever were.

Thank you for the feedback.