First thing I noticed (and was annoyed about) is that you guys don't use PSN messages to invite friends.
You know, the way almost all new PS3 games work. It automagically sends a PSN message to your friend who can then join your lobby/match by just selecting 'Join' from the options below the message, regardless of what he's doing. See COD: Black Ops and Killzone 3 for examples.
It should, of course, still be an option to accept invites via the dedicated tab in the in-game multiplayer menu of course, if you're already there anyway.
Also: Currently, you don't even get in-game notifications when you get an invite.
Another feature that would be convenient would be the ability to join your friends lobbies by just going into the in-game XMB's friendlist, press triangle on a friends name and select 'Join player'. Especially when you're in single player and suddenly decide to. See Modern Warfare 2 for an example.