When you are participating in a Lync meeting with 75 and more participants the experience will change to optimize the experience. The client will remove the participant list (roster), the gallery view will not be available and IM errors will not be displayed. The client will display the notification shown below.

In the title line of the conference window we will show the number of participants in the meeting. The presenters in the meeting will still get the participant list, such that they can take actions on the attendees.

We will not re-instate the normal user experience, even if the number of participants drops below 75. The experience will stay the same until the conference has ended.

The above change in user experience is not directly related to large meeting support in Lync 2013. It happens regardless of large meeting being enabled in the conferencing policy.