So what is its strong point? What can it offer me that Battlefield or Borderlands can't?
Well, if you're talking about multiplayer, it's an arena shooter, for one, which I'd argue makes it pretty unique in terms of AAA shooters with large player bases. Battlefield is heavily class-based and "realistic," and the only multiplayer Borderlands has is co-op.
Campaign-wise, I always liked Halo for its (slightly hokey) story but what I like most is the freedom Bungie always gave players in firefights. They were typically pretty wide-opened affairs that never played the same way twice due to the great enemy AI. They were like puzzles, especially on the harder difficulties.
I'm hoping 343i will continue that tradition.