For me, the main motivation is fluff. I love it - the sacrifice of Sanguinus, the struggle to contain the rage (& thirst) - all of it.
Upon my return to 40k, I had a plan, & the Blood Angels were not a part of it. I was going to make a small, fun army of Soul Drinkers then follow that with a small (again) Thousand Sons force & then do a "serious" vanilla codex (or DIY) chapter.
Plans changed when I turned to the BA codex for ideas for my Soul Drinkers. Admittedly, there was only a small amount of fluff in the .pdf, but it started me off & now not only have the Angels made it onto the list, but I'm starting them before I've finished the Soul Drinkers (I needed a break from purple anyway...).
I'll probably not get around to the vanilla marines now (BA will be my "serious" army), but I won't lose sleep over it. I'll most likely have a table-ready force by the time the new codex comes out - only to find half my guys will be illegal or trumped by some new uber-unit...