Core

This is normal behaviour. For the font options to be enabled, the cursor must be inside some element or some text must be selected.

In the case of alignment, the cursor or the selection must be inside a block element, because the option does not apply to text or inline elements.

Hmm...It is just confusing that you cannot set these values (even when you already wrote one long line of text).

After writing the first line, when I press Enter, I get the text wrapped in p tags and the selectors become active.
What about inserting a standard tag (a p tag?), if the RTE is still empty?
Then the styles could be applied there.

This is a difficult issue. The problem is that browsers do not render empty paragraphs. In the RTE, you cannot position your cusor in an empty paragraph. That is why the RTE transformation puts a non-breaking space in them. Therefore, we would have to do the same when the RTE opens empty. But I don't think that authors would like to have such a leading (or trailing) space before entering anything else.

When editing in the RTE, you can create empty paragraphs by pressing ENTER multiple times. You can see them only because the RTE puts a br tag in each of them. But these br tags are removed as soon as you save or toggle to text mode.

Would it be possible to insert a p tag with one nbsp in it and then to automatically replace this nbsp, when the author types the first sign into the paragraph?

There would be no way to know if the space was not inserted there by the author.

Hmm... But didn't this work with TYPO3 4.3.x?
Let me guess: The way it was realized there is not compatible with extJS?

Well, it worked the same way, except maybe that the toolbar was not correctly updated.

Perhaps we could initialize the content with a br inside a paragraph when the RTE is opened with no content and RTE.default.disableEnterParagraph is not set. Will need to do some tests.

The issue is very old, does this issue exists in newer versions of TYPO3 CMS (4.5 or 6.1)?