Why does Gmail website hide options like "links", "smileys" etc. under a '+' sign when there is clearly enough room along the footer? Am I missing something, or is google really pushing google+?
If you had 20 links, it would take you longer to scan for the attachment button than it would with two links and an expansion button. They are just optimising the most common use case.
You should always optimise for very common actions over rare actions. How much more common or rare they need to be is a judgement call which should be based on data and testing.
I'm not arguing whether this was a good or poor choice, as I don't have the data to make that judgement, but the reasoning is sound.
I do not see how the + sign has anything to do with Google+. :)
It is just a matter of balancing the importantance/"commoness" of actions.
By each thing you make directly visible, everything else becomes a little less visible. Google simply pushes the simplicity factor to the edge in their visuals, like always.
For example, it might be faster for some to find the smiley if it was not hidden behind the +. But it would take longer for others to find the more important/common attach button, if all other buttons, like the Smiley button, would always show.
You could ask the same question about a lot of things. For example the Formatting Options A button, or the More button in the top bar of google pages.
I find it successful. This balance is what makes it possible for my 83 year old grandmother to be able to find the attach button.
It's seems to be inspired by a Mobile UX where the UI streamlined to what is essential. I think it's a great example of UX patterns convergence.