What makes many of the oldies great are their design elements. Many of them simply had a more solid (imo) design philosophy. Nostalgia need not apply.
Personal refutation of nostalgia: Although I did play some of the classics back in the day, others I did not get to play or read. And, collecting them now, I judge them to be superior to modern offerings. YMMV. Also, I never played OD&D (3 brown books). But I've been paying them lots of attention lately, and am coming to the provisional conclusion that they represent a superior game even to my "favorite" version, B/X.
Elements of the oldies:
1. Difficulty. They were tough, and your characters could die in numerous ways. If you endured and triumphed, you could rely on that meaning that you did a good job.
2. No story. Most of the classics were detailed locations (one or more of dungeon, city, castle, lost island, etc.). What drew a particular group to the location was a rather open question in many cases. The emphasis tended to be on mounting an expedition and swiping the loot. The G series is an exception to this, having a more "epic mission" feel. I never played it. However, the method of approach still appears pretty open-ended.
3. Non-linear. Most of the classics are not just linear progressions through a series of areas. As discussed elsewhere, an excellent dungeon generally has multiple paths to the same place and does not require or even suppose that the party will "clear" an entire level, much less the whole complex. Dungeons, as a general rule (with attendant exceptions, of course) ought to be complicated.
4. Rules context. OD&D, Classic and 1E share a philosophy about the rules: hard and fast rules are few, and each DM is expected to handle his game differently. Of those 3 I mentioned, Classic is the most elegant and requires the least DM intervention, but also has very few actual rules. 1E is just a toolbox of rules. OD&D is a foundation, or something. Just to completely mix metaphors. The point is, within these contexts, a given module is more free to take on a life of its own, rather than conforming to an established and consistent set of rules and standards.
That's my take on it.