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.
Even there it wouldn't really compare, something that will be evident with VGER and DS9 too. ModernTrek was shot on much faster and less-rich looking stocks than TOS, and that is going to be even more evident with the best transfer. Look at some gorgeous Ted Moore shot pic like MAN FOR ALL SEASONS, then look at pics from the late 70s that are available light or with highspeed film ... no comparison.