Note that I split all of the replies about the Blood Ravens rules into their own topic, placing that topic in the Index Astartes forum (where the Blood Ravens are discussed). You can find the topic here. Since different Chapters are discussed in different forums, this topic should focus on the general series of Index Astartes articles and how they might impact the hobby.
A great gateway use of the Index articles would be for GW to include appropriate Kill Team rules (for relevant factions). The Elites book introduced sub-faction rules such as Chapter Tactics, So having the appropriate adaptations for the sub-factions covered in articles would be an easy way to get players using them at small scale. This would more likely lead to players expanding their collections to use full WH40K armies. And if they also included relevant Apocalypse adaptations, we might see more huge armies of the relevant sub-faction. GW hasn't done this for the sub-factions covered via Index articles to date (e.g., Crimson Fists - they didn't even include the Crimson Fists' Chapter Tactics in the Elites book ). I'm doubtful that they've done this for the Blood Ravens, but I'll be delighted if they do have Kill Team/Apocalypse rules included with the article.
As far as potency of rules go, I'm fine with the Index articles just giving us an interesting variation on a theme. I don't think that any sub-faction should suddenly become a must-have army. As long as the sub-faction remains comparable to the parent rules (e.g., in the case of the Blood Ravens, Codex: Space Marines) while being interesting to collect and play on the tabletop, that's mission: accomplished.
By extension, all of the same principles could probably apply equally to other sub-factions, but our focus in this forum is the Adeptus Astartes.