Wasn't Enterprise aired in high definition format? I don't own Blu-Ray, but how would it improve on something if it's already hi-def?
It's been aired in HD, but it's only been released in SD on DVD (and VHS). This release allows us to own it in HD (albeit with up-converted sfx) for the first time. It's a more marginal improvement than the other Trek series' but it's an improvement nonetheless.