There’s nothing that can make setting up VPN (Virtual Private Networking, which allows access to your network from other networks) truly simple, but Lion Server includes an L2TP VPN host that tries very hard - flip the switch, set a password, and determine what IP addresses will be used for connecting clients. By default, it takes 30 addresses from the high 200s, addresses that are unlikely to be in use on a small network. Make sure that your IP settings won’t conflict with addresses used by local clients.

The Wiki service is similarly simplified in Lion, at least as far as Server.app is concerned - you can turn it on/off and manage what users can make wikis, but that’s just about it.
The meat of the Wiki service is accessed via your web browser, where users with the appropriate permissions can both create personal wiki entries and create new general-use wikis.
I’m not a particularly authoritative source on wiki software, so I’m not really comfortable comparing the Wiki service in Lion Server to other Wiki products, but I can say that the Lion service seems to do the job reasonably well as long as you're not doing anything too advanced. The appeal for a small business is that Wiki is a simple-to-setup service that can host easily-edited internal documentation, or perhaps information and progress reports on ongoing projects, or maybe even meeting notes - the service is there to use, but as always your wiki is only as good as the information you put into it.