The section should show only the themes which are most downloaded
That is what the "Most Popular" section is for. I think you are missing the point of the "Featured Themes" section. It isn't meant for pretty themes or popular themes. As @Ipstenu said, it is to showcase different types of themes:
- Nice designs: there are several of those each time the list is refreshed. Currently you have ButterCream, Hatch, Patchwork, Esquire etc.
- Powerful frameworks and solid code: WP's default theme will always be here because WP wants people to use clean and well written code.
- Interesting concepts: Around a year back there was a theme called Quality Control, which presented the capability to transform WP as a ticketing system (the developer later pulled his theme). Currently you have Annotum Base, which does pretty unique stuff.
Just have a look which designs are on the top in the section and they are not even "featured" (picked) by an admin - delicate, twentyten and twentyeleven are on the first places because of regular Wp updates (of course)
Oh, you are very very very wrong. TwentyEleven has had only 3 updates in its lifetime and it is currently featured. TwentyTen has had only 4 updates and it was previously featured. Both TwentyTen and TwentyEleven have a high download count primarily because they are a part of every WP installation. Delicate has probably had around 5 approved releases in its lifetime and truth be told its stats look very weird, because there are stretches spanning a few days when it gets 50-100 downloads, then it suddenly shoots up to consistently 1000+ (without any new release). It is almost like people suddenly stop liking the theme and then start liking it again.
that if some themes which are not featured in the section with screenshots, they wouldn't have even 100 downloads a day, as their design (in my eyes) is out of date.
You are really making things up here. Design is not the be-all end-all of themes. Neither is basic adherence to standards that every repository-hosted theme has to comply with. Some themes are just supremely powerful in what they can do - if you activate Thematic you will see nothing visually striking in what comes out of the box. But Thematic was for a very long time in the top 15 popular themes list (without being featured and without frequent updates), because of just one reason - it is an awesome framework. The News theme is based on the Hybrid Core framework, and again, apart from being one of the nicer looking "magazine" themes, its strength is in its foundations. Toolbox is awesome when you want a starter kit for building an HTML5 theme for yourself.
And only if the section is built by real stats, then you (we) will know which designs matters.
Is the average theme rating a good enough "real stat" to show which design matters? Since you keep talking about popularity, just check out the average ratings of the themes you have a gripe against: Toolbox - 4 stars over 37 ratings, News - 5 stars over 25 ratings, Suffusion - 5 stars over 235 ratings and Easel - 4 stars over 20 ratings. Also check Thematic (4.5 over 147 ratings) and Hybrid (4.5 over 92 ratings) - these weren't featured, but they are probably the most powerful code frameworks amongst free themes. Users have rated all of them higher than Delicate - 3 stars over 10 ratings. Obviously the so-called boring themes have something that makes them tick: they all do the task they are intended for very well.
Unlike you I am not suggesting themes be made featured based on average rating - you already have a list for that, and the featured list is not a popularity contest. Think of all stats as box-office numbers (popular themes = weekly earnings, lifetime downloads = all-time earnings, average rating = IMDB rating), and think of the featured list as an expert's picks from various categories. That is the simplest analogy I can think of here. You might not like the expert's picks, but citing that he should change his picks because of your dislike is amusing.
The point is, if you want pretty themes, there are always places such as ThemeForest. If you want more than just pretty themes, come to wordpress.org, and the featured list will give you a preview of what a theme can do for you.