Some observations of the seniors on my team I really respect:
Patience in many forms. Patience to read docs and understand beyond surface level, to not jump to conclusions, to think about a problem abstractly before diving into implementation, etc
Strong communication. Whether translating a difficult problem to juniors, management, or non-technical folks. This also includes a significant amount of EQ (i.e. they're just nice people). Plays a role in mentoring and also arguing "up" for support on engineering priorities.
Willing to let go. This is a harder one to grasp - but basically being okay with scratching a project or area of code when it makes sense to, willing to set aside an opinion or lose an argument when it makes sense to, etc.
What else?
A senior engineer sees the big picture more readily, and understands the negative as well as positive impact of a change.
In any decision there's seldom one "absolutely right way". Usually it's a case of which compromises are the easiest to live with. Being able to see "both sides" and being able to see beyond the immediate is a key attribute.
Here's a hypothetical example - a Junior programmer wants to switch the development team to using Linux, because "there are lots of advantages to doing this". A good senior would ask the question - "ok, tell me why we _shouldn't_ switch?". There are, of course, disadvantages to switching (just like there are advantages and disadvantages to any decision). To show that you understand _both_ sides of the decision is critical. It's not enough just to know the positives, you have to understand the negatives as well.
In my experience you will get all kinds of advice to "absolutely do this". But when quizzed the reasons for _not_ doing something have not received the same level of thought. Folks who can see the big picture, and see the long-term impact of a decision, make better decisions.
[Of course the junior may be right, in the sense that it may be a good thing to switch. the outcome of the decision is not my point, it's the consideration process that I see as a differentiating factor.]
- Dealing with ambiguity.
- Diplomacy skills. Knowing how to negotiate.
* Ability to learn and use new technologies and methodologies and not be stuck in one box
* Ability to get things done/finish something or hand onto someone who can
* From above, delegation skills would be good!