1. Top quality industrial design and components (trackpads, screens, etc.).
2. Reasonably competitive specs (never as cheap as building an equivalent PC from parts, but close enough that you didn't feel totally ripped off)
3. An OS that provided Unix compatibility with a nice GUI that Just Worked.
Now, in all three areas, they've either fallen back, or their competitors have caught up.
1. The design and build quality of the best ultrabooks matches that of the Mac, and individual components like screens are often better. Meanwhile, Apple refuse to introduce real touch-screens on their laptop, presumably due to internal pressure from the iPad division, so they settle on the touchbar as a compromise. Or they want better sound, but can't make it work internally, so they just add fake speaker grilles instead.
2. The specs have stagnated, while the prices have increased dramatically.
3. The OS has gradually been locked down, and its Unix roots eroded. Meanwhile, Desktop Linux has improved, and Microsoft has added Linux compatibility to Windows.
The thinness?
Early use of UEFI, and PCIe and NVMe SSDs?
The first use of the Core 2 processors in anything was the MBP, which had been PowerPC before.
Plus all their interconnecticity protocols.
Seems reasonably bleeding edge to me. Completely fair to hold them to it.
Their computing platform has effectively been left behind: from five-year-old chips in laptops which haven't received a substantial update in almost as long, to their flagship prosumer "desktop" PC which is as unremarkable as it is unexpandable, nothing seems competitive anymore, let alone the bleeding edge upon which a lot of their products used to be positioned. The fact that the UE from their phones and tablets is being lifted and shoehorned into OS X (whoops, "MacOS") at the expense of usability and stability is more than a little off-putting, too.
It seems like all they care about these days are their mobile phones and tablets, and the iTunes universe that goes with them. I couldn't care less about any of that.