I've run G1-3, converted to 3.5. It was kinda crazy, the original modules are so big and have heaps of enemies, especially G2 where I had to trim out about half of the monsters etc. and even then they were close to being a real big grind.
So from my perspective, especially when Giants are Huge in 5e, the originals definitely need some form of editing for 5e. Personally I hope they just keep the old maps and text, it was compact and functional. Using Curse of Strahd as an example, the new maps there were actually harder for me to read then the original maps; while some interesting additions were made to the Castle, mostly it wasn't all that necessary and just added to the page count.
Some aspects of G1-3 were really cool and should convert to 5e OK, especially some of the "hoards of minions" aspects in G1 and G3 (the very beginning of G3 was pretty epic when I ran it - the PC's lured out a hoard of bugbears or suchlike, then used spike growth or something to kill them all as they ran down the entrance corridor - classic clever high-level play).
Definitely loads of opportunity for TPK, as well as totally "inappropriate for 5e" treasures - the fight against a pair of White Dragons in G2 springs to mind - my players were caught somewhat unawares, in the open, and quickly got slaughtered - so out comes the ring of wishes and a player wishes for a second chance at the fight... they had a good tactical discussion and prevailed on take 2 (just)...
As for the other adventures, I've played or DM'd most of them - like they are all very "dungeon crawl" oriented, but do have lots of variety and each offers some interesting points of difference. Reading between the lines, I'd say they are chosen so as to make it possible to play them all, in order, as a "super-dungeon" campaign, although personally I can't imagine that going down too well.
Lastly, I'd say that a member of their play-test group, it's interesting that we haven't been asked to test a draft version of this book, whereas for example I did get a draft version of Curse of Strahd before it was announced let alone published. As such, I guess one could speculate that the "conversion" of these adventures will be fairly "light touch", which personally I think will be a pretty good thing (for example: the 3.5 version of Cusrse of Strahd was pretty poor IMO as it totally changed the flavour to a combat-fest, whereas the 5e version preserved that original flavour and just added lots more interesting preamble).
