imo, the prequels lost all sense of charm from the originals. i think it may be the CG, and the fact that the acting feels so incredibly forced, but there just isn't a comparison for me. i watched attack of the clones (which i considered to be my 'favourite' of the prequels when i was younger, probably due to the action sequences), then empire strikes back straight afterwards and there was, literally, no comparing the two. ESB just feels so much better - it may not look as nice (subjectively, i still think it looks brilliant but the visual effects aren't up to modern standards), but it's so well-made and 'real', far more so than the prequels with their dismal visual effects.
maybe it's because i've grown up quite a bit since i last saw the prequels, and the films aimed at kids don't appeal to me so much, but star wars was originally meant for the youth of the day, and it still appeals greatly to many adults today, so i don't really see that as valid. the new films feel clinical to me, even in the action scenes that are meant to be dirty and chaotic. additionally, i just found it hard to empathise with anakin in the climactic battle, after he loses his arm, whilst i actually felt sad and scared for luke after he goes through a roughly analogous experience. it's hard to explain, really, a lot of it is based on just the 'feel' of the films. but, the express purpose of these forms of media is to make you feel what the creator intended, so they just failed at an elementary level for me. unless lucas was aiming for apathy or dislike, which i somewhat doubt.
that and fuckin' jar jar, and all the cringeworthy humour that they tried to use; it felt sanitised. the original trilogy had some snark and wit, with better delivery, whilst the prequels felt much more engineered. perhaps because lucas exerted more control over the prequels, he may have kept the actors more in line and allowed for less improvisation or deviation than the originals. what springs to mind is the 'i know' line from han, just before he gets frozen in carbonite - a completely improvised line. there just doesn't seem to be anything like that in the new films, something that you don't expect, yet fits the situation and the character to a T. and i don't even want to touch the story, there are many plotholes that i'd have to re-watch the films to have to rediscover, and i don't feel like self-flagellating today.
this is all superfluous, though. as many have stated before, watch the red letter media reviews of the series (here's the link to the one for episode 1 http://redlettermedia.com/plinkett/star-wars/star-wars-episode-1-the-phantom-menace/ ), he does a far, far better job than i ever could in a deconstruction of the series.