Others have already covered a lot of major changes that can be of great help. I have a some suggestions for a stylistic shift that would make me interested in buying modules again.
Don't Tell a story:
Part of the problem is that the modules AREN'T telling a story. They are presenting threat, detail its location and the combat encounters that will be encountered there. But that's not a story, that's just a collection of challenges. All of which are potentially deadly (so no one encounter is more important than another), and all of which end in a confrontation with the Bad Entity behind the whole nefarious plot.
On the other hand, explicitly telling a 'real' story in an RPG module is a difficult endeavor. Many times, the major NPCs are the real heroes of the story and the party are nothing but bodyguards who traipse along to fight a few threats and bear witness to all the interesting things the NPCs experience. This was the case in Ravenloft 2, the Randal Morn trilogy in 2e, but also in many Paradigm modules for 3e (in one of them, the PCs are literally the bodyguards of a senator who has to make difficult political choices - and I'm not meaning to slam Paradigm, I think they made Arcanis into one of the most interesting game worlds around. But many of their adventures are very disappointing). And the trilogy which started the Iron Kingdoms line also suffered from the same problem.
Another thing I dislike very much (though YMMV quite a lot) is all these dungeon crawls which have pages of background info which are a) pointless to the current plot b) cannot be discovered by the characters c) tell an exciting, intrigue-heavy tale which is now over and done with and leave the current PCs with nothing more to do but kick down doors and slay whatever monsters are behind them. The whole DCC-line was proud of its lack of NPC-interaction, which I personally find deplorable.
I think many of the modules written for the original West End Star Wars game are good examples, though. They are cinematic, generally put the characters in the driving seat, start with a slam bang opening which is often really exhilarating, and generally offer enough non-combat scenes during the adventure to keep the experiences offered by the module varied and exciting.
Same thing applies to the James Bond modules, several of which even improved on the plotting of the films (Live and Let Die, Dr. No, View to a Kill...). They read well and played well.
More recent good work can be found in the last Conan mega-module (Wrath of Asgard???). Many Elric/stormbringer modules are quite good. And for D&D, the UK series for 1e had some pretty good modules, the B/X module was very, very good (as were X4 and X5 for the Expert set). And Aaron Allston's introductory module in the N-series (where you started with 0-lvl characters marooned on an island and came to choose your class through your actions) was also exemplary.
Now, most of these are IP-driven lines, and the adventures try to approximate the feel of the original works. Perhaps that's one of the key elements - try and reproduce the excitement found in the original source material. And I don't think that source material should be CRPGs (based on D&D's own paradigm, anyway, so the snake is eating its tail), but instead good and/or classic heroic fantasy and sword and sorcery stories, novels and (a much rarer breed, alas) movies.
Maybe take a look at some structural models used for screenwriting (Hero's Journey may be very appropriate, or Blake Snyder's Save The Cat) and figure out how to use them for RPG module storytelling purposes (with one major caveat - instead of one protagonist who generally changes through and because of the adventure, you now have a group of protagonists you have absolutely no narrative control over). These models will in any case help you think about the overall shape of the adventure, the story reason for the encounters, and give them different weight.
And oh yes, ditch skill challenges for anything except purely physical activities (like escaping from a collapsing temple during a volcanic eruption). They're a truly wrongheaded design choice, a pain to read and obviously bloody difficult to design as well.
