I know this thread is a little old but Blu-ray's extra capacity was what I think really appealed to most people. I also like the idea of the required protective coating (whereas with HD-DVD it was optional), although blu-ray's data was closer to the edge and more susceptible to damage from scratches anyway.
The region free idea was what I really liked though. Blu-ray only has 3 regions but I'd rather it have one.
The extra size has yet to be used well on blu-ray, with many films still coming out in the horrific MPEG2 codec and you need about 100 GB to make that look as good as 20 GB of VC-1 or h.264.
The reason it was optional on HD-DVD is because unlike Blu-Ray it wasn't designed like an open sandwich, if the layer of coating wasn't applied to Blu-Ray the slightest scratch would screw the whole disk up, HD DVD was better designed in this area.
The region system really screws over Anime fans in the U.K, we could at least easily get Japanese discs before hand, but we can now get neither American nor Japanese. It's just hard work and very anti-consumer.