UPDATE: From a person of impressive large law firm experience:
Often - especially in large law firms -- partners with diminishing, diminished or little business, but whom the firm wants to keep for various reasons, take on bar association or administrative work at the firm. (The analogy in academia would be to tenured professors who do a lot of “service,” but do not do much research/publishing or active teaching.) There are many factors involved in such career paths (and certainly some highly distinguished lawyers who are truly competent and have significant business also take on such work). For associates, such a path is often (but not always) a sign that there is a problem, whether with the availability of work for that associate, the quality of the work, or otherwise.
By way of background, I worked for five years as an associate at a 100 lawyer office of a "large law firm" in the midwest. For the following three years I worked at a bank and became aware of the internal politics of two mid-to-large Chicago firms.
My perceptions of the managing partners at these firms is that do not tend to be "the best" in their field. That is to say, if they are a litigator they are not the best litgator at the firm; if they are a transactional attorney they are not the best transactional attorney at the firm. I think the reason for this is the best litigators tend to be the ones who love it and if you are a managing partner you don't have much time for litigating. My experience was also that managing partners tended to have very good relations with some of the firm's largest clients and/or a strong presence in the community.
The managing partners I was aware of were universally smart though and had many skills which many other lawyers lack. Most notably, while everyone likes to get what they want and many lawyers are good at getting what they want, the managing partners I was aware of were good at getting what they wanted through consensus or the appearance of consensus.
Assuming she has those skills, it would be interesting to see how they translate in a group of nine.
I was unimpressed with none of the managing partners I came to know of. Then again, none of them were nominated to be on the U.S. Supreme Court.
When I see that someone is managing partner, I can be sure of one thing: that person is politically astute and powerful. However, I would never draw any conclusions (based on that fact alone) about that person's pure lawyering ability or intellect.
What does it mean to be politically astute and powerful? It might mean that person has the largest book of business and the firm would fold if he or she left. Therefore, that person has the brute power to get anything he or she wants. Now, it may be the case that the managing partner has the most billings because he or she is the best lawyer in the world and all the clients know it, or it might mean that he or she inherited the book of business from a retired partner who was cultivated through years of sucking up.
The fact of being managing partner might also mean that the person has impressive people skills to unite a group of strong-minded partners. A related phenomenom is the managing partner who is the least objectionable among warring factions within the partnership. Of course, these qualities again say nothing about legal acumen.
Thus, a person can become managing partner for a variety of reasons, some of which may be related to intellect and lawyering skill but some of which have nothing to do with those qualities.
As for the bar activities, I would agree with the poster with "impressive large firm experience." When I see that kind of resume, I adopt a presumption that it may be a negative signal. Now, I would characterize my presumption as "easily rebuttable" because I have known excellent lawyers with significant bar activities. However, I have also known big firm partners who have used such activities to mask the fact of extremely weak abilities.
Bottom line for me? I just don't know enough about the nominee to form any judgments about her lawyering skills or intellect. I think it's admirable and impressive that she was the managing partner of a major firm, but it tells me nothing (without more) about intellect and analytical ability.