It is easy for leaders to get so caught up in seeing the weaknesses of others that they lose sight of their own development needs.

This situation is impossible.  If you are not excruciatingly aware of your own development needs you are not a leader.  (Although, to be fair, there are leaders who have no development needs - as soon as they identify one they focus on meeting it, so very quickly they don't have any development needs any more.  Until the next one comes along.)

If you are somebody who is fully aware that they are not a leader, but would like to become one, then self awareness is something that should be high on the list.

Also, leaders should not be concentrating on the weaknesses of others: they should be concentrating on their strengths.

If you are not sure what your development needs are, then as a starting point ask yourself if you can do the job of everybody in your team better than they can:  also all of your superiors and all of your business contacts (e.g. if you deal with salespeople from suppliers, do you understand the pricing model used by the salespeople at least as well as they do themselves?)   Also, you must have read Machiavelli's 'The Prince' at least twice. (A good translation will do.)

Every time somebody utters a sentence to you, and you don't think "Ah ha! I know exactly what to do here!" then you've identified a development need.