If there is one thing I don't like about Lightroom 4 it is this: the color space. As you are probably aware there are three main color spaces - ProPhoto RGB, Abode RGB, and sRGB. For the most part, and definitely if you are starting out, I recommend that you should stick to the sRGB color space as it's pretty universal. Using this means that your photos will look beautiful on your monitor and on your prints without you ever having to worry about clipping any colors because your color space is too large to be replicated on screen or in print. (In some cases you may want to work in Adobe RGB, but only if your chosen printer accepts it. Also, some people choose to work in ProPhoto RGB and soft proof different copies for web and print - the idea being that you edit with all the colors available in your master edit should you ever need them in the future - but for the likes of me this is just far too much. I don't work for National Geographic, I take snaps of my kid) So, in case you haven't guessed, I use sRGB for all of my editing, and as I mainly shoot people, skin tones fall well within the limitations of sRGB so suitable for my purpose.
For those of you still with me after that, in Photoshop, you are able to change the color space you are working in, either globally (so that ever photo that you open in Photoshop is converted to that color space) or for each photo. This way you can edit the photo in the same color space in which you are going to use for the final output.
However, in Lightroom, the color space you are given is Melissa RGB. The gamut (range of colours) in this color space is apparently is similar to that of ProPhoto - basically an extremely large color space which normal monitors and most printers cannot replicate. This means that you photo will have clipped areas when you go export it in sRGB. Here's an example of what I mean - this image is ready to go into Photoshop and I have the clipping warnings in Lightroom activated. No areas are showing up so as far as Lightroom is concerned, no areas are clipped.
Here is the same photo but with the soft proofing feature checked and my output as sRGB. See all the areas that are going to be clipped?
Now, this photo is too bright anyway (did it deliberately to show you) but it just shows that if you don't soft proof you may be clipping areas that you think are still OK within Lightroom.
Usually, I would simply go and change the color space so that I can edit all my photos in sRGB and never have to worry about it again, BUT Lightroom will not allow you to do this! I know it's no big deal, you simply check the little box that says soft proofing and edit your photo with this box checked, but I'd really like it if I could just change the color space and be done with it!
Not sure if there was anything to be gained from this little rant, but I feel better for it.
p.s If anyone who has upgraded to Lightroom 5 can tell me whether this does now allow you to change color space I would be very grateful!