1. Quick reply. I can't see any downsides/disadvantages to including this. Most forums now include this feature, it's very handy and afaik isn't a major mod. Simple as that.
2. Gamertag buttons. For people who own multiple consoles, it's a pain trying to fit both/all of their gamertags into the currently alloted. I've seen it done on different forums where there are separate fields for each tag, and at the bottom of each post, next to the Profile & PM buttons there is a button that is linked to the person's profile card.
For example, you could have a button like this that opens a small pop-up window: [url=http://gamercard.xbox.com/fourzerotwo.card:36pbvqxf]*Here there would be an Xbox Live button*[/url:36pbvqxf]
And you can do the same for PSN.
Downsides of this? Having to design a few little buttons and add them to be displayed under each post. Nothing major.
3. I know I'm most likely not going to be able to get an actual answer on this one, but any idea why the forums aren't upgraded to phpbb3?