TNG would look pretty bad in HD. It looks bad enough on DVD, very blurry, it's hard to describe but you all know what I mean. Hell, I'd say the unremastered versions of TOS are crisper (though certainly far more scratchy and grainy). Would love to see them do a good job of it though without having to redo all of the effects (unless they can keep them true to the original effects, unlike TOSR which I kinda disliked).
It only looks bad because it was posted on low-res NTSC video, so it would be very surprising if the TOS DVDs didn't look better. If they could go back to the original 35mm negatives, TNG could look amazing.
The issue is cost - full seasons might be prohibitive, but maybe they could test the water by doing Encounter at Farpoint, Best of Both Worlds and All Good Things (or whatever) in the style of the Fan Collective DVDs. I'd love to see what it looks like anyway.