This pack is named The Secret Armory Of General Knoxx and features some much tougher enemies and new areas to explore, although quite how many new areas to explore is not specified by the description. What is specified by the description is the level cap increase: previously capped at 50 it has been raised to 61. So it's been turned up to 11 - as the description on the steam store exclaims:
Borderlands goes to ELEVEN! You may have seen some games raise their level cap by 5, or maybe even 10, but with The Crimson Lance Assassination Squads wreaking havoc and Crawmerax the Invincible being all invulnerable and stuff we knew you'd need more than just 10. And that's why Borderlands goes to ELEVEN! The Secret Armory of General Knoxx adds new guns, new Class Mods, new vehicles, new missions, and increases the level cap by ELEVEN.Which I thought was quite fun, although I'm not sure I see a resemblence between the pictured General Knoxx and any of the Spinal tap band members?
Presuming this actually is General Knoxx of course, I'm just assuming from the screenshots available! Anyway I've not even started the first zombie-based dlc they released months ago yet so it's unlikely I'll be getting this straight away but it certainly sounds more like what I expected their dlc releases to be like.
More info is available on the Gearbox homepage