I have to say I'm not too interested in BGs, for that I can play BF3 or whatever. I prefer PvP servers where you can attack people wherever, and where winning or losing means something.
Warzones (what BGs are called in SWTOR) are instanced, team, objective-based PVP with around 2-3 groups (8-12 players) per team. One is a "conquest" type map, another is an "assault" type map, and the last is an interesting take on capture the flag that is kind of a cross between rugby, American football, and ultimate frisbee...only with booby traps and light sabers. While I do believe you get "valor" points for generic open-world PVP (which can unlock higher-level PVP gear), you don't get any (maybe as much?) PVP currency with which you can buy the PVP items.
That said, there are specific open-world PVP areas that are not instanced and do have objectives that do have specific rewards. These areas are PVP-enabled in both PVE and PVP servers. Additionally, there is one area on Tatooine that is completely open-PVP -- anyone can attack anyone there, even on the same side -- and there are specific (and unique) rewards for that area as well.
And also...does that game lag badly if you've got 20 people fighting? 30? 50? etc.
I didn't participate in any open-world PVP, but aside from a random 20-30 second server hiccup a handful of times throughout my entire beta, I never noticed any lag. This included in warzones where the entire team from both sides were in the same area duking it out.
And another thing...on PvP servers...if say, planets have to be triple instanced, that would seem to present a problem for hunting down your enemies. They might be in an instance of the world that you can't get into.
Note that "instanced" is used a bit differently than a traditional MMO "instance" in this case. In the traditional sense, an "instance" is a copy of a specific area (usually a dungeon) that is only for your specific group/raid. While there is that version of instancing in the game (both for dungeons and story-specific areas that are instanced for your specific stage in the story), there is also the ability to do zone-wide "instancing" which is sometimes called "sharding." In other words, the entire zone is copied and players are (randomly) assigned to a specific shard. However, you can at all times see the number of shards and you can switch to a specific shard through the UI. I am not sure how often you can switch shards, but I do know that if you join a group and are not in the same shard as your group leader, you will be automatically prompted to "move" to his/her shard. Therefore, if you are hunting a specific target, you merely need to find out what shard he or she is on, and you can make sure you are in the same shard.