In addition to the new cover illustrations (which I love, and for me are reminiscent of the Betsy-Tacy books, which are set slightly after these; it must be the pompadours!), these books are bigger - trade paperbacks, rather than mass market paperbacks. The best thing about the bigger size for some of us is the bigger print! As you can see from the link above, there have been many different covers over the years, but the one pictured above is the one I had as a child. It is very "well-loved," isn't it? I don't know what size the other versions are; do you?
I haven't had a chance to check, but hopefully these new editions also correct the typos present in the old versions, especially the appalling, and appallingly frequent, example which appears twice in the single page to the right. Can you find it?
I do wish that the publisher had added interior illustrations and possibly introductions by contemporary writers who themselves read and love the books, as in the new editions of the Betsy-Tacy books. Alas. They are certainly still worth purchasing for those of us whose copies will likely not survive being passed on to the next generation (see photo above!).
These new editions are published by Sourcebooks, a publisher I've never heard of. Have you? It appears that Sourcebooks has republished only the first five books of the series; I hope the rest follow soon.
Interestingly, my daughter prefers the older books - whether purely on aesthetic grounds or because she had already developed a sentimental attachment to them I am not sure!