Stress and workload - Project managers don't always do the best job of managing client expectations, leading to long hours and quick turnarounds more often than not. In all, it leads to a fairly stressful workplace from time to time. Part of this was my specific project (some clients are more difficult to work with than others), but it seemed that this feeling was shared by a number of associates.
On-site work - The firm is moving more a more to a system of on-site work and support for their clients, whereas before more work was done out of our offices. In the vast majority of my work, I was a support contractor for my client on an extended engagement, rather than working on quicker projects with a clear end deliverable. As a result, I often felt I was working within two separate corporate hierarchies, with 5-9 superiors providing (occasionally conflicting) feedback.
Quiet office - This will likely change when the Evanston office moves to a more open location, but during my time at ZS, the culture was very work-focused and quiet. People were busy, and as a result, they kept their heads down and stuck to their work. In addition, the increasing on-site work led to a more deserted office. The occasional social event/happy hour wasn't enough to cancel that out for me.
Fractional staffing - everyone complains about being on multiple projects at once. Even if you're not on multiple projects (I was normally on just one project), you have to deal with it indirectly, because all your teammates are fractionally staffed, and they only have 4-8 hours a week to devote to the project you're spending 45-60 hours on.
Working with India Offices - Associates often need to work with offshore teammates to speed up project work and cut costs. In fact, the on-shore associate role is more and more becoming a role where you manage the work of India associates. This creates an odd work dynamic where you almost never work with colleagues of your same level in your own office. Instead, sometimes you never meet face-to-face the people you work most closely. In addition, the time differential leads to work calls at odd hours (10 PM or 7 AM) and often leads to communication breakdowns.