My first post here, and I can't think of a better one. Forgive any typos as I am on my iPhone.
As part of my old job, I reviewed all seven DVD seasons fir IGN DVD and even visited the DVD production house when they were working on the final DS9 seasons. As a previous poster stated; TNG was shot on video and as such it doesn't translate too well to DVD. The existing DVDs are the absolute best the existing TNG elements can look on standard DVD.
Even SciFi HDs TNG broadcasts do not look much better due to the source elements.
The reason TOS looks great in HD is because CBS Paramount did a very costly HD remastering mission for the shows 40th anniversary. All of the effects were re-done with CG at HD resolution, and the source elements were cleaned up using the original negatives.
Now, TNG was the best selling Trek series on DVD by a very large margin. There is a RUMOR going around that CBS will remastering the series like TOS. If so, the BluRay will look great. However until then, the DVDs are the best you can get.