ITunes 10.4 on Snow Leopard failing in almost every way possible

Question

I just updated to the recently released iTunes 10.4 on my Mac, OS X 10.6.8 Snow Leopard.  I made the mistake of not cloning my drive just before the update, like I usually do.
The new iTunes launches just fine, but is unfortunately almost completely unusable.
Here is a running list of the ways in which I have thus far noted its failure.
1.) iTunes fails to recognize my iPhone 3G, resulting in the inability to sync apps, music, podcasts, photos, data, etc. It fails to show up in the "device" list in the left hand column of the iTunes interface.
2.) iTunes fails to recognize my iPad 2 with similar resulting problems noted above. It likewise fails to show up in the "device" list.
3.) iTunes attempts to download updated podcasts to which I subscribe as per usual. Cool.  They completed download never registers within iTunes. The download status bar moves all the way to the very end where it then states "processing file", and stays there perpetually. The downloaded podcast never shows up in the podcast library within iTunes. If I quite iTunes and then relaunch it, the process will be repeated with identical results. However, I have also noted that if I navigate to where the actual downloaded files reside in Finder, I can see that they have, in fact downloaded, and I now have multiple copies relative to how many attempts were made to download them within iTunes.
4.) Music/Audio CDs are no longer recognized in iTunes and thusly cannot be imported in the traditional manner. Although they show up as mounted on the desktop, they never show up in the iTunes interface and cannot be played through iTunes.
5.) Playing videos is tweaked beyond the telling of it. This one category of problems requires separate sub-entries:
     a.- Playing videos, movies, TV Shows, video podcasts, etc can only be initiated in iTunes when the video file is selected through the List View. Double clicking on a video file within Grid View or Cover Flow will not initiate video play. The one exception is within Cover Flow. A video can be selected in the Cover Flow pane, and the 'play' button can be clicked in the upper left corner of the interface to initiate video play. Otherwise, the video must be selected from the List View.
     b.- Once your video begins playing, you are pretty much committed. You cannot exit out of this playback unless you quit iTunes. Video Playback automatically takes up the entire "full screen" view utilizing the entire iTunes interface pane, no matter how else you have the video playback preferences designated, either in iTunes preferences, the View menu options, or right-clicked video playback options on the video playback itself. The video playback has taken over, and you're pretty much stuck with it. You cannot collapse the video playback with the little floating circle 'x' in the upper left corner of the video frame.
     c.-  The floating control bar in the lower third of the video frame no longer functions. Keyboard functionality is disabled, so pressing the space bar no longer activate play/pause of your video. The only way to play or stop the video is with the play icon button in the top left of the interface.  But even if you stop the video, it's remains paused at full screen - even if it has played to the final frame of the video.
     d. - Once you have initiate video playback, or even completed video playback, you can no longer see or access any of the other elements within the iTunes interface. Your only hope at this point is to quit iTunes, and then relaunch to access other content.
6.) The iTunes Store is no longer accessible. When clicking on the iTunes Store listing on the left-hand column of the interface, the interface window appears as though it will attempt to open your access to the iTunes Store, but it will not go beyond a large, blank, white interface window with the word "iTunes Store" in the middle of it, and the invitation in the upper right corner to "sign in" in small print - which, unfortunately will do nothing when clicked.
As a means to fix the problems, I went back to Apple and separately downloaded the iTunes update .dmg, and re-installed with it, outside of the standard Software Update method I first used. Not fixed.
I rebooted the system and repaired permissions multiple times.  Not fixed.
Additionally, I deleted my iTunes preferences. Not fixed.
On advice from another suggested fix, I deleted the iTunesHelper file from my Login items and manually re-installed it from the updated application package.  Not fixed.
I've scoured the MacFixit forums, and these Apple forums, and while I've noted numerous references to folks having problems with iTunes 10.4, I haven't seen them nearly as comprehensive as I've experienced them today, but I hope someone out there has a suggestion as to a viable fix that may restore the functionality I enjoyed this morning, before applying this circus of an update this afternoon.
I'm technically savvy about using my Mac and OS X, and if these problems stem from some type of user error, I admit I just can't imagine what it is or what I did wrong (aside from stupidly failing to clone a backup prior to update - allowing me to revert right back to the previous version of iTunes.)I would suggest that iTunes 10.4 update is a total dog, but that would unfairly insulting to dogs.
Thanks for reading.

Answer 1

Very many thanks to you, Putaro!  Your answer provided the solution. It was a buggy plug-in that turned out to be the culprit.
You're off the hook, Apple, and my apologies for stating that you let a dog run loose. You skated by this time, Apple. But I'm watching you.
But again, Putaro's reference to the plugins I had installed over the years turned out to illuminate the problem. I have several plugins in my iTunes plugins folder, pretty much all designed to add more visualizer functionality. I removed all of them, and ALL of the problems I noted in my post went away, with full iTunes functionality restored.
I began adding the plugins back to hopefully single out the culprit. Among the ones I had been using included fielder, G-Force, Timestrech, volcanokit, QuartzComposer, and Jax. I hadn't used any of them in a while - probably at least a verison or more back in iTunes updates, but iTunes continued to work with all of them installed. Currently, only QuartzComposer continues to work in iTunes 10.4. The rest no longer even show up as visualizer opitons, with one exception. Jax does show up, but does not work. AND THE PRESENCE OF JAX IS WHAT KILLED MY ITUNES.
I looked into it and went back to the developer, and it appears that they discontinued developing Jax some time ago - evidently, it was buggy and crash prone. I remember after buying it, having problems with it - having never worked to its touted potential, but I didn't pursue it, and never uninstalled it. Leaving it in there caused all iTunes functionality to cease with the iTunes 10.4 update.
If you've got Jax installed, consider removing it before updating to iTunes 10.4.
My iTunes is now working right as rain, thanks again to Putaro!
-Jon