Why a fixed line length?
Good question. So, here we have three columns; a main column (as shown in the teal background), and two gutter columns (as shown in the red background).
You wouldn’t want the teal too far to the right, otherwise you wouldn’t be able to see the right gutter. The avatar gutter (left, red) and main posts column (center, teal) are statically positioned while the right gutter is dynamic. The question as to why they decided on implementing it in this manner is for the developers to answer.
Why use flat design?
Using a flat UI has a better advantage over a skeuomorphism UI (and not just because its trendy or “in”) and heres a good reason behind this.
Its easier to customize due to the fact that there are no shadows (
box-shadow, etc.), absolutely none. There are no gradients. Everything is kept to a bare minimal which optimizes flexibility for designers. It’s quite a pain going in and removing a ton of unnecessary properties you really didn’t need. Now the necessity of whether you actually did want those properties will now be up to you the developer / designer as to if you do want them added into your design. I’m a lover of both flat and skeuomorphism (if implemented properly) design. If I’d have to choose one, it would definitely be flat or semi-flat.
Why no signatures?
Signatures are already implemented.
Click on your avatar located on the left of your post and you will see a signature The signatures are on your “profile card” when you hover.
Please be more specific with your question. You may be attempting to ask why are there no signatures within the posts. From my experience as a UI/UX Designer, allowing users to have their signatures embedded in the actual topic generates unnecessary “noise” while reading. There are other reasons why there are no signatures within the posts but your topic focuses on just design here.
Why is the avatar format the way it is?
Again, please be more specific with your question. What do you mean by format?