A brief commentary on expansion packs and the effects they have on established multiplayer gaming communities.
Comment: While expansion packs are great and add playability to games that otherwise might be getting a bit tired, I think it is safe to say that the multiplayer community looks at expansion packs a little warily. On one hand, they add a bunch of cool new stuff to battle away with. On the other hand, games like Quake, Half-Life, and every game built on even remotely decent engine technology has this ability built in through game mods. I think multiplayers are coming to expect add ons like this to be available for free, rather than having to shell out more dollars for an update.
The worst thing though in my opinion is that it leads to increased fragmentation of the community for that game. Not everyone will want, or even be able, to get the expansion pack, which instantly means there are two camps for this game, one playing the old, and one playing the new. Server admins have to divide their resources to cater for this, and ultimately we tend to see the communities shrinking even further as players start drifting away now that their game of choice has been fragmented.
What I would like to see as a solution is expansion packs that are backwards compatible. That is, instead of basically becoming two entirely separate products (as most games do), come up with some sort of system by which the new expansion pack version becomes the standard server for multiplayer games. People still playing with the older version, however, can still connect to these servers, but can not access all the functionality of the expansion pack for their own game. They can see the other players using it, and still get exposed to it, but they simply can't use it.
This would be great, as it would mean players that haven't yet bought the expansion pack can still play the game with those that have, meaning you don't suffer the instant community split as people migrate to their respective servers. It has the advantage for the publishers/developers in that the users that haven't upgraded will still be able to see the effects of the mission pack, and will hopefully be thinking "Man, I wish I could do that/use that unit/use that weapon/perform that action/get that item", or whatever.
There are a few problems, of course. There would need to be a system in place to differentiate between users, as least as far as online play goes. CD key authentication should carry this, but on LAN there would be nothing stopping players from just getting the mission pack and using it there, except the "honour system", which doesn't have a great usage history insofar as gaming is concerned. There would probably also be social problems - elitist attidudes by people that have the expansion pack generally just acting childish and abusing people that don't have it.
The main problem though would of course be balance issues - developers would have to make sure that the game maintains its balance when intermingling new elements with old. It would be pretty silly to make expansion pack users have a hugely significant advantage over non-expansion pack users. However, most games cater for this instinctively now, so hopefully it would be considered fairly carefully before implementation. I don't see any problems with a small advantage for expansion pack users though - again, the more incentive you give people to get the expanion pack, the better.
From a server perspective, as soon as an expansion pack is released, you're likely to simply see server admins just dividing the resources they have allocated for the original game into two rather than simply add more servers, for the reason that you're unlikely to suddenly get more players overall - just players from the old version checking out the new. Depending on the pickup, allocation numbers could work either way - if the expansion pack isn't very popular, I can imagine server admins not even bothering to put up a server if it means putting owners of the original game out of servers.
In the long run, the above solution to the expansion pack problem works best for everyone - gamers and server admins. The same resources can be used for all the gamers, meaning less division in the community, and more servers available to everyone.