These new remasters' most consistent strengths are enhanced clarity and stereo separation/imaging. Individual albums (and tracks) have other positives as well - tight, accurate bass, good high-end extension, excellent presence on Plant's vocals, and so on. Some notable examples:
- The Lemon Song (Zep II) sounds better than it ever has before, particularly the tightness of the rhythm section.
- I Can't Quit You Baby (Zep I) - Bonham's drums sound amazing, right in the room with you.
- Tangerine and Bron-Y-Aur Stomp (Zep III) - super-clear, veil has been removed.
- Hats Off to (Roy) Harper (Zep III) - less harsh than other versions, and clearer, actually makes this a song I enjoy listening to for once - if you don't love the sound of Page's guitar in the left channel, then I just don't know what to say.
- Custard Pie (Physical Graffiti - available in CD-quality lossless from Qobuz online) - first time ever that this song doesn't sound like crap (sonically, that is - it's always come through as a great performance).
- Pretty much all of Physical Graffiti and In Through the Out Door are sonically best-of-breed, including all prior vinyl versions IMHO.
On CD, a lot of folks love the original 1980s CDs for their dynamics and non-aggressive mastering. I think Barry Diament's original 1980s versions of Zep I, II, and Houses of the Holy still have a lot to recommend them. But I think the rest of his CDs have been superseded by these new remasters - as have all of the Marino remasters from the 1990s. And even with Zep I, II and Houses, the Diaments can't touch the clarity of the new versions (not Diament's fault - he had to use the tapes they gave him, and he was working with 1980s digital technology).
Also, I was fortunate to stumble into test pressings of the remasters of Zep III and Houses (main albums only, no bonus discs), and they sound very good. I haven't had a change to hear them on a truly good turntable, or to clean them properly, but from what I did hear, I think folks are really going to like these, in some cases even better than the digital versions. I should note that I wasn't super-impressed with the Houses vinyl - nor with the digital version; it might be my least favorite sonically of the new remasters, nothing wrong with it just seems to lack that certain "spark" of excitement in the sound. But the Zep III vinyl is going to be a real treat I think!
Finally, I should note that some people feel these new remasters are a bit too hot on the high end. I disagree - I find the high end extended but not fatiguing. The most controversial so far on this count has been Zep III - because of all its acoustic material and the "meh" quality of the original recording/mix. If you are concerned about this, I suggest spending $1.29 and purchasing "That's the Way" off Zep III from the iTunes Store. It's the worst-case scenario for treble-averse listeners. If the opening acoustic guitar strumming - and particularly the sound of Page's guitar pick - sounds prominent but not objectionable, then I would say you will be extremely happy with the entire remaster catalogue. If you immediately think, "Ahhhh my ears are bleeding from that guitar pick!" then you might not love the Zep III remaster (though Zep I and II have gotten almost universal acclaim). But I would say go ahead and buy the Zep III deluxe edition CD - the bonus disc is the best of the three, and IMHO it's worth the $13 to $15 all by itself.
Oh - and to answer a question above, Zep IV/Zoso and Houses of the Holy are slated for October. No other Zep reissues will be coming out until 2015. I would assume Physical Graffiti (and maybe Presence with it) would come out around Feb-Mar, but that is just speculation on my part.