The difference between the classic red box and the new one is, the original just has more game. This isn't really about the level range, but the support for that level range.
I believe, this perception is influenced by the fact that you got the original red box when you knew nothing about what else is out there, but today you are comparing the new red box against all possible available material.
Try building 5 characters using the new red box. You'll tear your hair out. I think the 'walk through a character creation' is great, but they should have included a few pages of a more standard character building process. Old red box had a 'how to play' walk through, but you could skip it and just go to the character creation section. Plenty of replayability, even with only a few basic classes.
I agree that having a sheet that tells you how to build characters without the solo adventure would have been nice, but I don't think building characters with these rules is as arduous as you make it out to be. It's not necessarily something you would do for fun, but it is serviceable. Moreover, the new red box gives you a much larger variability in the characters you can play.
34Second, how many monsters are included in the new red box?
How many magic items?
7, not a whole lot but magic items are less of a focus in the new edition.
Does it have tips on creating your own dungeon?
6 pages plus 2 pages describing a world to set the adventures in. It even has suggestions how to reuse the original map to create new layouts.
Classic Red Box was a game. It can be expanded, and often was, to either the Expert box or AD&D. Essentials Red Box is a demo, with limited replayability. For that you need the DM Kit and a Player's book.
I can say when I was young I only had the B1-9 collection, and I don't think I played D&D beyond 3rd level until 3rd edition. (I did wander off to play other games including Palladium Fantasy, Warhammer, and Talislanta in between.) I still remember each of those old adventures fondly.
You can of course do the same thing with this box. If you look on the DDI, there are tens of adventures you can play with this box, as well as the excellent Slaying Stone. And as 4ed adventures are self-contained, you don't even need monsters used in the adventure to be in the box.
I'm not saying the new red box is perfect, the rules inconsistencies with the Essentials line is very annoying and the large number of editorial errors makes this less useful to beginners than it could be, but framing a product that by itself allows for tens of hours of replay as a demo is unrealistic. Moreover comparisons with the classical red box generally do not generally account for the fact that the first RPG will always look different; thus they overrate the classic red box.