Since there has been debate on what a spoiler thread should or shouldn't include...
It's been established that at the very least a small number of CMers would prefer to have conversation about topics without being subject to spoilers about publicly unaired material. Please make sure to utilize one of the spoiler buttons or tags (spoiler & sblock) when discussing anything that could be considered such. It's why they are there.
To take this a step further, putting a [SPOILERS] tag in the title doesn't give enough definition. All this means is that what has been made available for public consumption will be discussed. Want to have a discussion where spoilers about unaired material is fair game and doesn't need to be (s)blocked? Label your thread accordingly so others are forewarned. Something like [FUTURE SPOILERS] sounds about right.
Speculation is fine but revealing something definitive you discovered about plot, casting and the like is potentially spoiling the experience for someone else if they weren't expecting it.
Bottom line: The point of a message board is to have discussion and encourage participation. The best way to act is to include as many people as possible and state the intentions of conversation up front.
Don't be a dick. Play it safe and be courteous to your fellow posters. It's not hard.