First off, I really like the Breiðafjörður Nature Reserve but you will see why I don't rate it highly soon.

I took the Viking Sushi Cruise from Stykkishólmur a couple of hours north of Reykjavík. It started ~10am and was quite full even on a windy and slightly rainy day. The main draw of this cruise is that it takes you to some of the nearby islands (I forget their names) for bird-watching and catches a net of clams. They even provide soy sauce and wasabi! There was also enough for everyone so it was probably worth the money.

The tour does not go as far north as Flatey island. If you want to go there you need to take a ferry by Sæferðir, the same company that does the cruise. I doubt this place is worth visiting for the settlement alone. It's a traditional fishing village which is not uncommon in Scandinavia. So for me the 2 first criteria are not justified. If you visit the website of the ferry company you can see an appropriate picture of the islands - one picture to see all, yes, it's that small. Might be a good place for bird watching and there is a hotel on the island.

The third criteria for the rich biodiversity is in my books always worth protecting and can be unique, but it doesn't sound unique, doesn't seem unique, and we didn't see anything special on this tour. We saw puffins (my first Icelandic word: Lundi - and now I know why the British puffin-filled island is named as such!!), lots of Iceland Gulls, European Shag and a few Common Eider. None of the rare species. Even if they are in the area I wouldn't find having rare birds in a nature reserve inscription-worthy. Just protect it at national level and that's it.

I recommend the cruise for a nice morning out though.