If you really mean level 1 and level 2 (and not Act I/Act II), the difference is that once you acquire the level 2 key, the second dungeon is unlocked. Prior to that, it's a locked door (or trapdoor/portal/whatever).
That is what I meant. I've read some of the posts and it has become clearer what the difference between the battlegrounds between LEVELS and between ACTS are. I hadn't explored far enough before I asked the question to realize that there are 3 pretty much separate outdoor/dungeon areas in each battleground act, and that they are progressively unlocked when you find the keys.
