I'm going to say a toss up between Nested projects and scripting.
I haven't got a huge base of other software to compare it to. mostly just NewTek's stuff, and that wasn't too impressive to me in terms of workflow and ability to do tons of editing stuff in a single app.
Spot, I know you're looking for a feature like what you'd list on a spec sheet. I didn't choose Vegas, nor have I stayed with it, just for one particular feature. However, just to play the game, I tried to imagine what Vegas would be like if someone took away just one of those features. Would the absence of any single one of these make me want to switch to something else? You have to do it this way, because the right answer is going to be something that is really essential, and probably something that has been there almost since day one.
My answer is pan/crop. I use still photos in almost every project, and the ability to animate them, and do it quickly, has let me produce some amazing things in very little time. I sure see a lot of posts from other people, especially those using the various commercial script add-ons, who rave about the ability to quickly create slide shows.
Now, if your real reason for asking is to go even deeper and try to understand what REALLY makes Vegas so special, then I think you have to get into the underlying UI and code structure. The original reason I bought Vegas is that it is so RESPONSIVE. I can experiment and create without waiting -- in most cases -- for even an instant. I can undo instantly. I can cut, rearrange, move, etc. without blinking an eye.
The engineers who did the original design, created a real thing of beauty.
XDCAM support. Vegas was the first NLE in its price range to be able to take both IMX and DVCAM MXF files from XDCAM straight onto the timeline right out of the box. In fact it may still be the only one since other NLE's such as Xpress Pro only take the DVCAM flavour of MXF.
I would have to say that the ability to drop almost anything onto the timeline without having to get it into any particular format is the most productive feature in Vegas. Also the ability to directly manipulate events right on the timeline is a close second i.e., trimming, fades, opacity, etc. without having to use menus, just direct manipulation. That is the core of what makes Vegas so powerful for me.
I am learning FCP 5 at work (because I have to) and cannot tell you how frustrating it is to see "UNRENDERED" when I come to a simple transition. I've been using various versions of Vegas since 2000, and to not have realtime previews (let alone having to transcode audio if it isn't just right) is unacceptable, IMHO.
Oh, good thread idea. The main reason I Switched to Vegas was the audio stuff. I was using Premiere to edit and Cubase to spruce up all the audio - The slow workflow was killing me, not to mention the problems with both those programs.
I downloaded Vegas and tried it out and cut my editing time by about a third. I could do all my audio and video goodness right there - in realtime! And it's stable!
The other killer feature is being able to dump anything on the timeline. I recently had to "save the day" by adjusting some audio from a DVD on site with my laptop. Had to dumux an ac3 file and re-render a vob file - not pretty but very usable and the client was very happy. When I went back home the editor (who works in FCP) was asking "how is that even possible!" - Vegas versatility has saved the day for me many times.
The absolute stability. Following close is pan/crop but after a marriage to Pinnacle 7,8,9 that was bad from the start I can't understate the feeling one gets from completing an edit without a freeze, dropped fx, crash, or losing a single bit of work to the alt/control/delete gremlin.