Hi,
Here are some thoughts on multi-player based on my limited experience with PBEM.
Maintaining Interest: In my experience, my interest level is the highest in games that progress at a stead pace. I'm in a game with 3 players and we average 1 turn per day. I'm in two games with 4 people and we average 2 turns in 3 days. I'm in a game with 5 people and we average 2 to 3 turns/week. I'm in a game with 8 people and we average less than 1 turn per week. Games can be delayed by many things: players out of town, turn sequence not optimized, players not serious, etc. I suggest that you try to get people that are motivated to play and willing to check their e-mail as often as possible. Ideally, all players would be able to check e-mail and play once in the morning and once in the evening if there is a turn waiting for them. This will ensure that you get in at least 1 turn per day.
Dedication: A PBEM game only advances if each player takes his turn and sends it to the next person. I'm not aware of a way to substitute a player with an AI. That means that if a player is getting killed and knows he will be out of the game in the next 10, 20 or 50 turns, he must still be willing to play his turns so that the rest of the players can continue to enjoy their experience. I would stay in a game that I was losing for no other reason than to be a royal pain in my attackers side. The game goes on and my bitter enemy suffers while trying to finish me off. If I can get others to dogpile on him at the same time, who knows, he may die before me...
Optimizing play order Strictly going by time zone isn't always the best alternative. For example, I can play at work if there is a turn waiting for me. So, I can pretty much play from 8 AM to 10 PM. On the other hand, if there is a player that can only play during a small time window (e.g. 6 PM to 10 PM), it is best to ensure that there is always a turn waiting for this player at 6 PM so that he gets his turn completed for the day. Arrange the players accordingly based on their availablity to play in order to make this happen.
Difficulty Level: Most players tend to think that difficulty only relates to how hard the AI is. However, it has other implications that affect game play as well. Changing the level of the player affects multiple things including beaker cost per tech, hammers per item built, happiness/health cap, etc. If you set the level higher, not only will the AI be more difficult, but it will take longer to reasearch techs, build things, etc. Since things are already slow in PBEM, I would steer toward a lower difficulty to increase the tech rate and increase the build rate. This is about playing other humans, not about facing tough AI, right?
Game Speed If you set the game speed too fast, by the time your units get to their target, they could be out of date. If it's too slow, the game will last years. Normal seems to be a good balance. I've never played on quick.
Map type Do you want a game that gets interesting quickly? Or would you rather go into builder mode where everyone is racing to complete as many wonders and found as many religions as possible (i.e. you don't like wars and conflict)? If you prefer the former, pick a map where everyone is on the same land mass. If you prefer the latter, pick tiny islands or multiple continents.
Tech Trading: The downside to tech trading is that a player can give away all his techs and change the game completely. The upside is that it adds more meat to the diplomacy aspect of the game. The upside of not allowing tech trading is that you get what you've earned. There is no free lunch. If you focus on building military at the expense of research, your 10 warriors will be slaughtered by 2 longbows. The downside of tech trading is that players that fall behind will have difficulty catching up.
Team Play: Civ is typically a single-player game (unless you play in some of the Succession GOTMs). Even when playing PBEM, there isn't much interaction going on between players as each silently takes his turn and sends it to the next person. There is a diplomacy aspect of the game, but the interaction between players is still minimal most of the time. Playing in teams allows the players to interact, discuss strategy, make war plans, etc. It's a great way to learn from each other and get better. The downside is that it can potentially slow down the game if players take a long time to play their turns due to disagreements on what to do next (I have not seen this hapapen yet).
If you get the right combination of players and game settings, PBEM is a lot more fun that playing against the silly and often stupid AI. If you get it wrong, you could end up investing a lot of time into a game that dies before it ends.
Good luck!!