Pan-Orthodox Vespers, as I have experienced it in travels across the US, has the same general feel every time I go. You often see the same faces over and over. They are the same people you see at local conferences, the same people who sing in pan-Orthodox choirs, that attend ordinations, etc.
Somewhere in the room is a noisy baby or one that likes to throw things. This has at times been one of my brood. Some people dress up and others are in t-shirts and blue jeans (in the South you will definitely see sandals). At the periphery of the sanctuary are the people (usually 2-3) who disdain pews and refuse to be near them. There is the person who is supposed to take pictures of the event and the 5-10 people who choose to augment the photographer's efforts with their own camera-phone portfolios - these photos pop up on Facebook or blogs that same night, which is a solid week or more before the official photos are posted.
There are never enough service booklets. The people that get there early give one to every member of their family and there is rarely a redistribution of booklets for those who come in later. Many people will sing along regardless of their possession of a book or familiarity with the music or translation of what is being sung. There is an amazing variety of wording in America for even the most basic prayers of the Church. I know 4 Our Fathers, 3 Trisagia, a different doxology for every parish I visit, and that's just the normal introductory prayers.
The service is often a shell of the full service one would expect at their home parish. Being an infrequently celebrated service, there are always a few moments of confusion in the altar when entrances or changes of clothes are called for. Most of the congregation doesn't notice and the rest bring it up as soon as the service is finished and they make their way towards the fellowship hall.
Usually a goodly number of children attend. Having attended a full Liturgy just hours ago they are raring to go outside and do something other than sit or stand in place in another room. That said, I have never seen a parish plan for the annual eventuality of gaggles of children roaming the church grounds. The halls are often filled to capacity with people and the temperature is just this side of sweltering. The food is, based on commentary I've heard, not Lenten enough or "we aren't a monastery" too Lenten. The bookstore is usually open and people buy all manner of things, many of which had not been touched in years.
This a good opportunity to see the variety inherent in the American Orthodox experience. For people who have only ever attended one parish many things they assumed were obligatory or "Tradition" are now visibly optional. Other things they never even pondered are immediately visible. If parishes are smart they'll bring flyers for their annual pysanky sale, ethnic festival, retreat, or other event because there is little chance other parishes will hear about them otherwise.
Looking forward to next weekend's pan-Orthodox event.