A major issues affecting WiFi routers today is P2P traffic. Many formerly happy WRT54GS users have had to upgrade as their Online Gaming / BitTorrent / Skype / VoIP usage has increased, as the cheaper lower-power devices can't keep up to the dozens, into the hundreds, of connections that these applications spawn under NAT.
I haven't seen Apple commit to any specifics on connection tracking, as have some of the devices like D-Link's GamerLounge series.
It would warm my heart to know that these excellent devices would also make for a great router for gamers et al, so I can turf my ugly D-Link..
PS - Also, I have been noticing an intermittent problem with the Airport Express "forgetting" about shared USB printers if they get turned off. It needs to have the printer reconfigured, even after a reboot in this scircumstance.