1) Stability.   It's hard to put a value on the extra confidence.
2) Automation Lanes.   It took a while to break from the in-track control, but I really like this.
3) Track colors.  Perhaps not fully baked, but so much better than previous X versions.
4) Quad Curve fly-out. 
5) Melodyne/ARA.   Already had Editor but it's ARA that cements the deal for me.
6) AD2.  Although it was AD1 included with X3 at the time, that is what got me to AD2.
7) Blue Tubes plugins.  
8) Comping.  I liked what I've seen the few times I've used it.  I still record most often in Sound on Sound mode but should really loop record more often because I think it would work well for me.  Old habits are hard to break.
9) Lounge Lizard.
10) Bi-filter.
 

The most:
Prochannel
Overloud TH2
Addictive drums
Automation
X-Ray windows
Audio snap
 
Not yet:
Melodyne
Screensets
Gobbler
Matrix
Take lanes (or whatever they're called these days) / comping

Previous SONAR X series compatibility, Melodyne...The legacy continues.... -S

Anderton
 But, this is still interesting because it's giving me some ideas for articles about the lesser-used features.
 

You've no doubt written a post on the use of Audio Snap but I've not sought it out.  Many forum comments  (along with my very limited experience) suggest to me that it is difficult to find it useful.  On the other hand perhaps it's usefulness lies in  the ability of the user to properly implement it.  I've tried AudioSnap to syncrhonize multiple audio tracks but somehow always seem to make things worse.   

AudioSnap can work very easily or be very ornery. Like ACT, there are simplified ways to use it that Bitflipper has alluded to. I think an article would be appropriate.

The audio snap feature was covered in another blog article way back when. I never could reproduce the results that the bakers did, so +1 for more info on that feature. If you're not too busy mixing drinks.

Not using colours.... (wich surprised me....)
 
Totally in love with the take lanes... 
Like the whole fatchannel layout... And it really saves time. And I don't use VST audio plugins that much anymore.
 
I'm still missing a good HUI implementation... Since I have devices wich use coloured knobs and so.... And ACT doesn't really explain how to use coloured lights.... 
 
The big thing I am eagerly wanting to test out is the touch screen thing..... I have a feeling that will be awesome with some rackmounted pc and some rackmounted mic pre's so I can easily do recordings here and there and show the power of plugin and be creative with the musicians instead of doing so on my own and coming back and like opening pandora's box.

My experience is almost identical to Craig's. Melodyne and comping are the huge ones and they alone made the upgrade to X3 worth it. I haven't really done much with most of the included 3rd party stuff, although I occasionally use AD and the BlueTubes delays and chorus. Mostly I have other stuff that I know better for these things.
 
It's not technically part of X3 but the CA-2A gets a lot of use from me. A feature I'd like to explore more is the loop construction window, I have no idea what that thing does, most options are greyed out or senseless (to me) and I've only ever opened it accidentally.
 
EDIT: Oh and I have used BiFilter, it's not a plug I need in every project but it is absolutely awesome for when I do need a filter. It has great distortions too.

Melodyne and QC EQ are probably the two things that I am really using.  I'd lump AD in there except that if it wasn't included I'd be using SD and that would be ok (although AD is much better IMO).
 
I am using melodyne as a creative tool - using it to convert lyrical melodies to MIDI for manipulation into hooks, fills and solos.  I can sing a tune, convert it to MIDI and they use the midi to feed a synth.  I have also used to tighten up lyrical melodies as well with regard to setting (what words end up on what beat) as part of the writing/arranging process.
 
Enough has been said about QuadCurve.
 
I would love to see an (some) article on loop construction.  Not just on how to use the tool but context on when and why to use the tool.  I've watched a couple of videos on it and can open it and push the buttons, but I am not sure I know what to do with it.
 
I haven't explored TH2 much or the Blue Tubes stuff, but that is more due to a lack of time spent with the software.

 
Comping- It was a little rough at first but I'm finally getting the hang of it.
AD2- Really like the variety and sound of it. I wish it used GM mapping but I finally figured out how to use the AD    drum map so I can live with it.
QC Flyout - Very convenient, and I'm using the QC a lot more since this feature was added
Track Colors - I know it's controversial but I like it
 
I already had a workflow going with Melodyne so the I don't use the integration. Might like it if I did but I'm used to the old way and it works fine for me so....
Don't use the tape sim, don't like the sound I get with it.
I have no need for the TH2 plug, and never liked it much anyway.