I have noticed that this is a frequent issue among younglings from technical areas such as ours.

In the beginning of our careers we simply don't know how to sell ourselves to our employers, and random guy #57 (who is a programmer, but not as good as you - technically) ends up getting a raise or a promotion because he knows how to communicate and market himself better than you. Many have probably seen this happen in the past, and most certainly many more will in the future.

What kind of skill/ability (either technical, or of other nature) do you think is relevant to point out when doing a job interview or asking for a raise, besides listing all the programming languages and libraries you know?

Get things done.

The people that have the power to promote you will only be impressed when they see results. Simply learning many libraries won't be enough to gain you any sort of promotion. It probably will, however, gain you respect from those immediately working with you.

Also, don't think of it as 'selling' yourself. It's a case of showing that you're worth your weight in gold; this can be done by making it obvious to the higher-ups that you accomplish great work and that you're capable of achieving many things.
