that's a good example, I would go even further and clone out the white noise spots under the wing (and in other places)
Persnally I don't have any problem whatsoever with levels, curves and brightness/contrast type adjustments, as these are things that would be done when developing pictures "the old way" and if it was acceptable then, why not now?
AS to colour adjustment - well here you are simply compensating for the way modern digital cameras calculate white-balance, which is often an imperfect science. In the case of old scanned negatives or prints, you are compensating for the effects of time on the original media.
With cloning out extraneous details, it's a little less clear. My rule of thumb is that if it's a techincal photo, it's quite appropriate as it is simply abstracting only that which is important to the purpose of the photo. However if it can't be done well then I suggest it be left as is.
However I also like to avoid becoming complacent when taking photos by thinking that I'll just clone out any odd details. 1. this creates more post work, and I hate work! 2. it creates a sense of laziness that in the end only makes you a worse photographer. Not everything can be cloned out. It is FAR FAR better to get it right when taking the photo. I reserve cloning for those times when it just wasn't possible to get it right at the time, or for those times I simply screwed up :
