Why don't I consider Lulu as a Metallica album ?
Because it's a collaboration. For me, a release doesn't belong to a single band if there are other bands-musicians collaborating on it. (I'm not talking about guest musicians, I'm talking about partnership) It's not Metallica - Lulu, or Lou Reed - Lulu, t's Lou Reed & Metallica - Lulu.
Metallica did some experiments back in the day, with Load, ReLoad and St. Anger. These albums were made all by themselves, songs were written all by themselves. I don't see Lulu as a "direction changing" nor do I see it as an "experiment". They thought "Let's make a album with Lou Reed" and that's it. This album was %50 Metallica, %50 Lou Reed. Actually, it's more of %60 Lou Reed, %40 Metallica. Metallica came up with riffs and solos. The lyrics are Lou's work, vocals are Lou's work, concept is Lou's work, the sound, the style is Lou's work. (if you look at his older works, you can see that they were on the same way as Lulu is)
And from where I see, Metallica doesn't consider Lulu as a Metallica album either. We all know they care a lot about money, they care a lot about merchandising. They posted the songs off the album online way before the album is released. Some say that was because the album was leaked, I don't believe it. Because Death Magnetic also leaked, Metallica put the release date back, it was their solution. Next album probably is going to leak too, they would't post the songs online because of that, they would put the release date back once again.
And I'm pretty sure that the next Metallica album is going to be on the same path as Death Magnetic, another thrash metal album.