The main one I had to deal with was display issues with the Retina Display. The zooming is lethargic, some parts of the text fields did not refresh in Safari, the cursors sometimes disappear, animated GIFs get garbled frequently, and there were video issues with Quicktime.What problems have you got with Mountain Lion? We've got several machines running that, zero problems. By far the best OS I've ever used.
I have heard of various others issues as well, but only had the one Mac to deal with.
I do not think Blu Ray is going anywhere for quite some time. The simple reality is that streaming an uncompressed blu-ray requires about 25-35 mbs. Sure you can compress the signal and get under that, but you will take a quality hit to do so. Downloading is a possibility, but it will take a good deal of room. Your average movie on blu-ray is in the 20-22 GB range. Add bonus material and you can easily be in the 30-40 GB range or more. That would put a movie collection of just 200 movies at about 5-6 TB. Doable but not common. Storage will catch up, but so will 4k resolution and them we are right back were we started.Well for Apple it is about iTunes and digital downloads. Blu Ray is a piece of plastic that will ultimately become unavailable, as will the players, and you will be left with a collection of drink coasters.