* Combining the simplicity of the classic MacOS Finder with the keyboard controls and hierarchical menus and customisable taskbars of the MS Win95 Explorer, without the horrid bloat of Win98 and all later versions, including copies such as KDE >=2.
* Removal and elimination of a huge amount of legacy bloat, making a tiny fast OS. (e.g. No text mode, no console, at all.)
* Removal of complexity of multiple languages and C as the lowest common denominator; everything is in C++ and nothing but.
* Richly multithreaded: ran extremely well on SMP machines in the 1990s, when this was strictly limited to huge workstations that cost as much as a house.
It was gorgeous. Haiku is lovely but IMHO a little bloated and sluggish by comparison.