I really enjoy watching other people's games, but I usually only ever watch one turn. Any more than that and I think you run the risk of getting drawn into the battle and inevitably taking sides. I like to see the standard of their painting and modelling, first and foremost. But I also like to see their army compositions and the tactics that they use.
I'll happily chat about the paint and modelling aspects of the armies on display, but I don't ever volunteer any tactics.
When I'm playing I'm happy to reciprocate and am more than pleased to talk about my Eldar or Space Wolves with whoever cares to stop by. But I will not tolerate a "Back Seat Driver" chipping away behind me, telling everybody what I should and should not do. I'll give them a chance though and ask them to stop, after all anyone can make a mistake. But if it continues to happen, I will ask the shop manager to remove them.
Though I take your life, I promise you the stars...