You are hearing the effects of poorly written code, bad dithering, and not very high internal bit processing.
Things you will notice (sometimes, if you actually KNOW the difference between even decent analog mixing and computer based) are the stereo field sort of shrinking inward as you apply more DSP, the "depth" getting shallower as you apply more DSP, and the sound getting "harsher" as you apply DSP. That nice "round" "warm" sound will start to turn very stale.
On a full on ProTool, Sonic Solution, or Sadie system, you will have far less of this. The internal bit resolution is MUCH higher than Windows current 32 Float system is, and the code for the DSP is much higher evolved in those systems than most PC software is.
When PC goes to a full on 64 bit Float, much more powerful software code can be written (still no guarantee that it will be written well....

) and you will start to hear these effects go away. BUT, you are going to need increasingly higher CPU speeds to keep up!
ProTools at least had it right to make the DSP Farm cards. They are like PCI cards that run on their box that has dedicated DSP, with the included software with them. In any of those system I listed above, the computer (PC or Mac) doesn't actually do ANY of the processing on the sound. The systems 'box' does all that, and any included DSP cards. A few companies making PC cards were trying to do a similar thing, but most PC's that are doing high track count recordings are using up the PCI slots for many other things the system needs, so there isn't much left for DSP PCI cards. Maybe one of two on the average system.
Yup, you get what you pay for. Amazing that somebody around here is actually posting something about this! Most rave about their PC software recording stuff as being just as good as analog and much cheaper, but having recently been doing some PC recording/mixing at home here, I am finding that it really lacks. Very slow to make changes while mixing, hard to configure to get high track counts, and it is VERY annoying to lose your last hours work because Windows ran out of memory!!!! If I see another Fatal Exception Error I just may do the 3 1/2 hour drive up to Seattle and strangle Billy boy Gates myself!!! Of course, I will have a long line to wait in....
http://www.digido.com has some excellent reading about that loss of depth in all digital recordings/mixing. Worth reading about if only so you sort of understand what is really going on.
Ed