Great topic and great posts thus far, here are some things I think are essentials...
1) Being Proactive versus reactive - you must guide your candidate and client through the process and talk about things like other interviews and counteroffers long before those things come up. Get up front contracts with your candidates and clients on common struggling points- if you have the same thing repeatedly costing you from success- you MUST learn and keep that from happening by proactively tackling that problem next time.
2) Ask the tough questions/your ability to stomach conflict - average recruiters accept answers that they want to hear- but does that really help? You need to have the courage to stomach conflict and make it productive, not argumentative. The tough questions are designed so you know if you are wasting time or not. Your ability to successfully deal with conflict and ask the tough questions keep your time from being wasted.
3) I am a salesman - not the slimy snake oil salesman type, but a professional, you need to ask productive questions that uncover all the pain and motivations from both the client and candidates, unless you do this you cannot successfully know what is best for the prospect and which direction to lead them. Also beware of your language- your words are just a small part, what is your tone and physiology telling them? It has everything to do with how you are perceived.
4) I do not need a weatherman to "feel the wind blowing"/ deciphering candidate and client talk (experience) - this comes with experience, well, some people learn and others do not, but many of you know what I am talking about, when a candidate says "your opportunity is at the top of my list" .......what they really are saying is there are many other opportunities that are better than yours, but I am afraid to tell you that. In which case you move to #2.
5) Constantly pursuing perfection in the client and candidate relationships - over-communicate, bombard, copy and confirm, sweat the small stuff, the details. Be accurate, thorough and timely. Think like the client and/or the candidate and care what they think and feel. Regardless if you think you did a good job, if they do not think you did- then you did not.
Added bonus- have fun, this business is about people - not money.