I've been watching demos of high-end synthesizers, like the Yamaha Tyros, Motif and others, all of which are powerful workstations, and was thinking if there's a computer software that has all this advanced stuff like simulating a real playing performance of an instrument, like the bass guitar performance where it sounds as if a real player strums and slides on the strings, or preloaded rhythm patterns, arpeggiators and so on.
In this case you would just hook up a simple 88-key controller or digital piano and it would operate as a top-notch synth.
I do know about fruity loops, but it doesn't look like a real synth (or does it?) to me...
Is there a program like this, that has that interface and functionality? (probably a really heavy and expensive one)

The computer software is far more advanced than what you'd get with a Motif or Tryos. But people still like the Tyros, Fantoms and the like because they are so well integrated with the keyboard and some people don't like computers.

I've been using "Logic" for a while now, not an expert at all. Bundled with Logic are about a dozen or so general purpose software instruments. One is a virtual analog synth where you can patch oscillators and filters and the like to create new and unique sounds. I think people like analog snths because they are "real", not a sample of copy of sme other instrument. Logic's synth is like this because there are no samples.

They also bundle in samplers and drum machines, Modelled guitars, basses amps and speaker cabs and about a dozen pedals, midi sequencers, music notation editing and printing, analog recording and editing, 1,000 instrment samples and loops. Controls for stage lighting and the list go on "forever" OK yu get the idea, lots more depth than any "workstation" and about 1/5th the cost.

http://www.apple.com/logicstudio/in-action/

Here is a list of some synths that are bundled. They all have on-screen mouse driven interfaces but to can connect these to some MIDI control surface and work them with a physical slider control, drum pad, keyboard or whatever.
http://www.apple.com/sg/logicpro/moresynths.html

There are several companies that do this.

You should check out www.Spectrasonic.com they do a few Vst's that fit your description.

There stuff is sample based. So taking from real instruments.

But it can get pricey.

Absynth is one of my favorite software synths.

http://www.native-instruments.com/#/en/products/producer/absynth-4/

You can make some really amazing, complex sounds from this software. I will happily browse sounds for hours in this program.

I also use a Moog. There is no comparison to real Analog!!! It just sounds very pure.

Thanks guys. I checked out Spectrasonics.net and really liked the Trilian, which is a virtual bass instrument. In the demo they show the different playing types, and you can actually hear how the "virtual fingers" are plucking\tapping\sliding on the strings. Amazing! Exactly what I've been looking for. Could software like this be plugged in into audio producing software (like ACID) or virtual synthesizers?