[Timo]

For me, it's its slightly darker character, and the velvet-like quality of the sound engine.

Also:

* Alias free synthesizing when using the knobs, it's completely zipper free.
* I love the quality of the EGs. The pitch/filter curves sound fantastic, not lifeless like many other VAs. If you actually try and emulate patches from the Virus on other VAs, they just fall flat on their faces when you try to emulate the Virus EGs. I think this is one of the most important aspects in why the Virus sounds so good.
* A very versatile filter section.
* The effects are quality. Nice sounding phaser, and the quality of the reverb/delays blows most other "built-in" synthesiser insert effects out of the water. I'm not so fond of the Distortion effect, however, but I still use it. That said, those lucky enough to own TI OS v3 have slightly greater control over the distortion, and also have new algorithms to add to the classic distortions.
* The detune parameter is slightly curved (logarithmic). You can obtain really fine (subtle) detune adjustments near to 0.
* The fact that the LFOs can be put into single-shot EGs is great. The available LFO waveshapes and overall LFO control are versatile too.
* The mod matrix is extremely well specified. You have control over almost everything.
* Unison mode is great and you have decent control over the parameters.
* Microtuning option.
* PCM waveshapes for a greater sound palette.
* A limited but nice FM implementation, that has made home to some oft-used classic Virus sounds.

.... and so many little things too many to mention that make up the whole experience.

And it's often hard to make it sound crap.