Was prompted by conversation with @Iszi on chat to make things much clearer - to just highlight the main increased risks.
An attacker could reroute every request sent by users of the network leading to:
Phishing attack - for example the normal guidance for users is to never click on untrusted links for things like online banking, but to always type in the url for www.examplebank.com. Controlling the router lets an attacker get around that control easily so you could end up at a site which will grab your online banking credentials as you log in.
Malware attack - You can be rerouted to a site which will attempt to download malware onto your computer even though you specified a known good URL
Inline data injection/modification - An attacker could modify any data in transit. This would be hard to do from the perspective of a user of the wireless network, but much more straightforward for an attacker if all data was routed via a server they control
Other attacks based on this Man In the Middle - various other compromises
The internet is your oyster with this one - effectively users would no longer be able to trust any communication through the network! It is almost as good as pwning the users computer.
The other points I mentioned are already possible on wireless networks so I have taken them out.