Played the D scenario once. It was pretty good, I think it'll be fun for hardened 1830 players to try something new, or for long-time players who haven't played it before to introduce the game to new players with. It didn't quite feel right to me with 4 players though - it felt like there were just slightly too many mid-game trains (3s and 4s mainly, perhaps 5s), and the classic endgame crunch takes too long to start - resulting in a slightly over-long and somewhat attenuated game. With 4 anyway, it means the winner will come from the player who starts and runs 3 companies instead of 2. I think with fewer players, it might help to remove some of those trains. Easing up a bit on the rough endgame in 1830 isn't a bad idea, but I think it pulled back too much. But again, I can see it might be quite different with 5 or 6 players.
The 'D' scenario does have a rather different feel from the classic scenario, which is good.
I also can't help but feel like the scenarios might be a little bit too much. What I would have liked to see is a minimally intrusive scenario that helped rebalance things a little bit and mix up the geography: some new or different private & public companies, some rebalanced geography, and that's about it. The new scenario chrome - coalfields, different certificate mixes, different off-board box rules, new tiles, new promotion rules - all strike me as too much. 1830 was great because it was spare, it did what it needed to do then got out of the way. It frustrates me the endless need to add chrome to 18xx variants (I particularly despise '+' trains) when simply changing geography and perhaps slightly re-parameterizing existing structures would be more than enough.
It's also slightly annoying that 1830+ has stuck to the anachronistic tile mix of the original, a mix that was defined by production, not gameplay issues. Some important plain-track green and brown tiles can run short in a very frustrating way. We always played with a double set of plain track (no extra cities or towns), because being screwed because there is only one green 'Y' track in the mix is silly (my '10' rating is based on playing with a doubled plain track mix, I might have to dock it a point if you play with the needlessly constrained mix that comes in the box).
Just got the new Mayfair edition, and I have to say that out of the box, it looks really great. I love the customization and scenario options. I bought the new edition because I am a big 1830 fan, but as I mention below, because I've played the basic game so much I wasn't sure I'd actually play the new one. But with the enticing scenarios I can definitely see this coming off the shelf.
One warning, though: the many variants enumerated can have really profound effect on the game, not all of them good. For example, the rules that allows 50/50 dividend splits and delayed obsolescence are both huge and not in my opinion in a good way. Tastes vary, of course, which is why they're there - but I would make sure you are familiar with the game and understand the implications before picking and choosing. Your intuition on what "makes sense" can definitely lead you astray. When I first started playing 1830 I played with the delayed obsolescence rule; when I finally played correctly, I thought "wow, that's a lot better", and understood why the original rule had to be. Anyway, the variants are just a smorgasbord of how people play 18xx, don't assume that because they are in there it means that they make sense or are a good idea (or work well in combination with other variants).
The sole exception I would make is to recommend the extra track tiles. I haven't done a comparison to the original 1830 manifest, but we always played with a second set of "plain" track because there are a couple configurations that were needlessly short (UPDATE: Sadly, the extra track does not address this issue, and there are some needlessly complicated configurations in there, so just grab all the plain track tiles and don't use the rest unless you're playing a scenario that calls for them).
My personal preference is for open corporate cash and closed private cash - I'm actually not sure this is one of the options suggested. I think this is the best mix to provide the key tactical information required to play well, while blocking any decent into needless AP. I think it's clear that given the game is what it is, open corporate cash should be the default preference. People will have different preferences on private cash though.
Awesome game, if one with a limited lifespan as it will be eventually "discovered", at which point some of the edge is lost. Nontheless, there is a very long gap between the first play and that point, esepcially if you wisely try to get to that point yourself rather than reading all the online advice
Another big win for 1830 is that it really isn't that long, about 5 hours after the first game or two, and is just the length it should be. This is one of the best of the 18xx games, and of the longer multi-player games in general. An important caveat, though, is that this game really can't be played with players of significantly differing experience with the game. For this reason, I think 1830 is essentially "dead" at this point for me, but I had a *lot* of fun getting there.