I usually judge plug-ins by two factors:
1) how they perform. Do they use a lot of CPU? Are they crash prone? Are they stable on SMP systems (*cough*waves*cough*)?
2) how they sound. This isn't an easily quantifiable factor, but more a question of do I like the way it sounds. If a plug-in diminishes the signal to noise ratio, that's not necessarily a bad thing.. it's just a quality of the plug-in. Likewise if an EQ's filter has ripple...
Having said that, if plug-ins and the actual recordings are the parts that define the character of the sound you are making, then the DAW itself can just provide a neutral platform for making that happen.. That's the goal, anyway...
We've joked around with ideas about having algorithms for analog "bleed" between channels etc... but it's probably not really a good idea.