I enjoyed Borderlands immensely and, as I'm sure you know, it shares some similarity to Brink in that its a co-op FPS/RPG hybrid.
The problem with borderlands is that even with all the DLC its really a short game My buddies and I tore through it all to max level more than once. more than a few times, actually. But in the end, we stopped playing because it wasn't fun any more. We'd seen it all, done it all and in a relatively short time. I think it took us three to 5 nights to do a complete playthrough from original content to and through all of the DLC.
Brink looks like a far better game and we cannot wait to play it.
My questions, though, are these: What are you doing to make the game durable? What will keep us playing for many months? How will restarting the game as a new character lead to a different gaming experience? How will the injection of DLC or expansions make the game feel longer or deeper rather than feeling like we've dipped back into the game for mini episodes?
These are just questions I'd love for the devs to ponder as they prepare what is sure to be a smash hit game -one I hope is a favorite for months or years to come of continued gameplay. If any devs do read this, please address this issue in a blog some day, many of us are very curious as to what direction you're taking in regards to this.