***The Good***
This book is mostly laid out very well and is clearly intended to on-board new DMs before throwing them into the deep end. It does an excellent job (there are few resources that do it better IMO) on explaining how to run a game, how to make calls, how to make rulings, how to manage players, how to create and run a session zero, how to handle common player problems (powergamers, murder hobos, etc.). Excellent chapters on how to create and run an adventure and how to create and run a whole campaign.
Speaking of campaign - the Greyhawk gazetteer is much bigger than I anticipated, it's very well-laid out and illustrated and very actionable (much better than Krynn gazetteer in the Shadow of the Dragon Queen for instance). It is sufficiently big and contains enough maps to be able to fairly easily run a whole hexcrawl in Greyhawk. The book also contains a very nice lore glossary - useful to new DMs. Likewise, the big chapter on the multiverse and outer planes is also surprisingly chonky and very actionable.
I like the new magic item system (with two big caveats - see The Bad later) and how to give out magic items. There are more magic items here than there were in the old DMG, with more illustrations, more ways to customize magic items, and more artifacts and relics.
I also like that this book contains several small adventures that can almost be run on the spot as well as maps to go with them. Again, very good for new DMs. These aren't like huge adventures but they are good to drop in as campaign starters or quick side quests to put in.
I like the new travel rules in the DMG though it still necessitates some book flipping and referencing PHB, but I think it's a better travel system than 2014 DMG had. Likewise, the chase rules - while not fundamentally changed - have more nuance to them now.
There are a lot of new things in the DM's Toolbox chapter (but with a big caveat again) that are quite handy.
The bastion system is pretty good (also with a big caveat) and I would mostly use it if I have a chance and a choice. It's not as super crunchy as some other home base systems I've seen out there, nor is it quite as abstract as some others. Overall, I really like this new addition BUT it feels like it should be in the PHB, not in the DMG.
Oh and the art is absolutely spectacular.
***OK, now for The Undecided***
Some magic items have received changes but they are somewhat hard to spot so I haven't gone through that section with a magnifying glass to find problems. There might be some problems there but it's too soon to tell.
The bastion system no doubt adds even more potential for powergamers but I'm yet to try to game it and see exactly how it does so.
This is a minor omission - it's not rustling my jimmies too hard but I would've liked to see this part of 2014 DMG included - the 2024 DMG includes a section on how to create custom backgrounds (which is great) but not on how to create new races or new subclasses (which the 2014 DMG did include).
The true encounter builder is not in the DMG at all, which I found to be a bit puzzling (there is a sort of encounter builder but see below in The Bad). They did mention that it will be in the MM instead where - according to the writing team - it makes more sense. However, there are some other crucial rules in the DMG that depend on the encounter level, so it would've been nice to see it here. Not a deal breaker, but would've been nice.
I think the book does not provide enough guidelines and advice and rules for NPCs - the NPC section could be beefier to help out new DMs.
***The (spicy) Bad:***
Still no firm and sane magic item prices - we were promised that this will be in the book, and it's not. It's still the same vague baseline chart copy-pasted from 2014 DMG. I'm actually mad about this because let's face it - the number one use of DMG in every edition was for the magic items and to not finally have firm prices here is ludicrous. And speaking of magic items and treasures - they completely got rid of the tables for randomly rolling magic items, treasures, and treasure hoards. There's no reason you can't use the 2014 DMG for this but this seems like a step backward.
Also, compared to 2014 DMG and previous editions' DMGs, a big step backwards in terms of settlement generation and rules - I find that section in 2014 DMG very useful and here it's instead been reduced to very limp two pages, mostly filled with tavern name generators, and some other random generators. Very lacklustre. The authors even say that they expect DMs to look at how the City of Greyhawk is presented in the book and use that as baseline but it's such a cop-out.
Likewise, big absence of random encounter tables by biomes (or maybe I just missed them completely). Maybe they will be in the MM, but at least some sample ones and some advice on how to create them, would be nice.
Another fairly important section to explain to newer DMs which should be in the How to Make a Campaign chapter but isn't, is on how to create pantheons. There are about two pages related to this but they are not very helpful.
The DM toolbox - while I think is more newbie DM friendly than the old DMG, could still be organized better. I'm glad it's much earlier in the book and is given more space, but it's kind of all over the place.
Then there's the encounter builder which is in the book, which is basically a "XP budget" style encounter builder. In my opinion this is even worse than the already broken encounter builders in the 2014 DMG or in XGE (or Tasha's? Can't remember). All it does is provide a bit of guidelines as to how much XP an encounter should be worth but it's useless in terms of providing any sort of guidelines about how to actually go about building an encounter. As a seasoned DM I don't really need it, but it's a huge omission in a DMG, especially for a newbie DM.
Finally, I like the bastion system but there are such obviously great options and obviously poor options for players that it makes me question whether any playtesting feedback was taken into account. If I was a bard with bastions, I'd feel punished.
So there we go: in most ways it's an improvement on the 2014 DMG, it's certainly a much better GUIDE to how to be a DM, but in other ways it's a bit of a step back. It doesn't have a lot of new things (other than the bastion system, the Greyhawk gazetteer, and some new tables and maps) to offer to a veteran DM.