When I make a DVD from HD material, I export my timeline using a High quality "lossleess" codec, e.g. the free download UT codec. This produces a very large file, but avoids a lot of problems with quality in the necessary re-encoding.
I then use AME to produce a DVD legal file for Encore. I create my DVD, and then I play it on a player which has Up-scaling on my Hi-def TV. Up-scaling players give much better results with DVDs than straight DVD players when you have a large screen TV.
The cameras you use make it sound as though you may be coming from a still photograph background. Well all the problems caused by lack of pixels and down-scaling you met there are present in video, but in spades!!!
The frame for 1920x1080 HD video has around 2 megapixels, while the frame for PAL SD video has round 0.4 megapixels. Think how many megapixels you would want in a still image to print at A4 size with adequate results, let alone a poster sized 40 inch sheet of paper. NTSC video has even fewer pixels than PAL!