We'd like to profess that the main rule is "have fun!," but when you're opponent gives you the stink eye for blocking through the net, you at least need to be prepared to argue that there is some ambiguity on the issue.
Indoor volleyball rules saga
Like any sport, volleyball evolved over a lengthy period of time and was subject to interpretations by many organizations who catered to the players they represented. A handy guide through the founding maze of acronyms can be found here. Mercifully, the rules are starting to converge.
Blocking through the net
You pass the ball to a teammate but miss a bit and knock it towards the net. Your opponent deflects the ball down by letting the ball push the net into his or her hands. If the ball causes the player to touch the net, it is not a fault, but should a team lose their chance to salvage a point due to what amounts to an intentional block through the net? The USAV changed their mind in May of 2008, calling this play illegal and labeling it a net fault by the defending team. Various other authorities concur, if the touching is intentional, it is a net fault. Otherwise, if the momentum of the ball causes the net to touch a player, it is not a net fault.
The "Let" serve
Perhaps it's old news, but this constitutes one of the major changes in the game in the past several years. The ruling on a serve that touches the net used to be that the serve would be replayed. As of 2000-2001 USA Volleyball indoor rules, as long as the ball crosses the net, it is playable. This keeps the three foremost defenders busy with more than prepping for an attack and maintains the game at a steady clip.