Mixdown: Is something lost, thus requiring Mastering?

I know what you are talking about. The project seems to sound one way but the stereo mix is not as good. I believe this is due to a mistake in the export settings used. Also some effects (plugins) may need to be rendered in real time. Not many though. 
 
One thing that might help is Mix Recall. Try different mixes using it and export each. 

Mixing is about finding the best balance among a collection of individual tracks. It doesn't matter if, for example, all the instruments are lacking highs somewhat if they all lack highs, because you can still balance them.
 
Mastering is about working with a single entity rather than a collection of tracks.
 
It's entirely possible to come up with mixes that need very little tweaking. But, you need to anticipate what you want the final sound to be, and build that into each track of the mix. Sometimes it's more effective to concern yourself only with the balance during the mix, and do the overall processing while mastering.

John
One thing that might help is Mix Recall. Try different mixes using it and export each. 

John, I thought Mix Recall was not available in X1. 

pinguinotuerto

John
One thing that might help is Mix Recall. Try different mixes using it and export each. 

John, I thought Mix Recall was not available in X1. 

True. I didn't realize he is on X1. Now I feel sorry for him.  

Seems I interpreted the question differently from others - that it wasn't about bouncing or exporting changing the sound, but about the difference between listening to a project for balance and listening for delivery.

bitman
This is a round and round that ends when you've had enough, "release" it and move on to the next. That said emastered.com is free right now.

gave this a shot yesterday because of reading your comment.
I must admit, it wasn't too bad. the levels compared similar (not quite but close) to my own masters.
(I did a side by side comparison of a master I did yesterday.)
 
As I said, the levels were similar, but the over all compression and EQ were a lot different.
I found emastering had over compressed the mix vs. the one I mastered was more limited.
I also found a generic EQ present Vs. a dialed in sound to fit the genre and the material.
but over all, it was free and it IS something I would use if I were in a bind and needed a quick master if I didn't know how or didn't have the time.
 
with all this being said, I have custom mastering templates that fit a different jandra of music that will do the same thing. I typically load those templates and then spend a few hours tweaking them.
 
thanks for the heads up on that lsb. it was an interesting experience much like Landr.
was interesting to compare the differences.

stevesweat
  To answer the first question - theoretically a finished mix should need no "mastering"; if it doesn't sound right yet then you are not finished mixing. Mastering originated as an art of placing multiple songs on one album and ensuring the songs have a sonic similarity and sit well together.
  In practice it is now commonplace to process in the stereo realm after the tracks are blended. People call this "mastering" but it is not mastering in the true sense. True professional mastering is always done in a studio dedicated for mastering and not by the person who mixed.

 
For a long time the most important aspect of mastering was tailoring the final stereo mix to fit the dynamic range and frequency response limitations of the distribution medium. You could have a perfect mix in the studio, but that didn't mean it would translate to vinyl. Although it was common to do different mixes for mono and stereo versions, from that point on, mastering took care of preparing the final mix for the varying requirements of LP vinyl, 45s, cassette, and sometimes even radio.
 
This is one reason why mastering engineers attained such a mystique: yes, they would sequence and level-match the album based on the artist's input, but this often had to modified based on constraints of the delivery medium. You couldn't put a loud song on the inner grooves, bass had to be mono, there was always a tradeoff of length/level/frequency response, and so on. The typical tools used to adapt the final mix to various media were dynamics processors and (particularly to avoid freaking out the RIAA curve) EQ. It was the mastering engineer who generated, and discarded, test pressing after test pressing because some of this process was trial and error as you tried different tradeoffs. Some great mixes have been recovered only since the dawn of the CD, when the original master tapes no longer had to compensate for vinyl...compare some of the recent Blue Note CDs to the vinyl versions, then try to tell me that vinyl "sounds better" 
 
You might think that with today's digital media, there truly is no need to master if a mix sounds good. But that's "in theory," and the world doesn't always agree "in practice." For example after decades of people being acclimated to compressed sound (overcompressing has been around since long before maximizers hit the world), dynamics is still used as part of the mastering process. Applying dynamics to program material produces very different results compared to applying dynamics to individual tracks while mixing. Also, EQ can do "broad strokes" while mastering that would be time-consuming to do on a track-by-track basis. It's also sometimes necessary to do separate masters for CD and MP3 because of how the MP3 encoding process alters the sound, and of course, a compilation can consist of great mixes that stand on their own, but don't flow as a collection. 
 
A really good example of the difference between mixing and mastering involved a project for a woman who was traveling to Italy and left me with a mix to master. She wasn't around for feedback, and I really couldn't tell if it was intended for more of a club sound or an ambient vibe. So I mastered the mix in two different ways. When she came back, she liked them both; they were both valid, even though they were from the same mix. 
 
Although in this post-vinyl (maybe!) age there remains much merit in having an independent set of trained ears adding the final production/artistic steps to a mix, being able to master is not mutually exclusive with being able to mix. It's a different skill set, but with enough experience, it's possible to be good at both. The hardest part is more psychological, namely being able to "forget" you ever mixed something, so you can be objective and treat the final mix as something you've never heard before. In the rush-rush days of putting out music, projects went right from mixing to mastering so you needed another person for the required objectivity. With today's more leisurely pace, you can put a mix aside for a month or two, and come back to it fresh when it's time to master.
 
In classical projects, mastering in the conventional sense has become pretty much superfluous. But for pop music, mastering remains not only the last step in the technical process, but nowadays, it can be the last step in the production process as well.