I was going to put together a long post about the relative aspects of 4E vs B/X and then decided that I wasn't going to use my time in that fashion. Instead here are just a few quick thoughts about the 4E game I ran a while ago:
1. It is rules-light - I don't know what proportion of page count in 4E books is rules vs powers (rules exceptions) but it is heavily, heavily, heavily skewed in the direction of the later. No DM can remember all of them. If you have a regular group then the DM can remember the 20 to 30 powers the party has. But for a walk-in game during a meet-up, you don't even know what races and/or classes will be at the table much less the powers. The players just have to tell the DM what their powers do. But the framework upon which all of these powers hang is pretty simple.
2. I tried to keep a semblance of narrative combat - and I was surprised how easy it was. Before the session began I took a look at everyones' powers - not for mechanics (see above) - but instead for the fluff. I made a note of some key points of the fluff about each power and used those to narrate the effects in combat. It was much more than "I move to here and use my Vorpal Viceration power". The player may say it that way but after the dice were rolled I gave the narrative of dodging through the combat and their shining blade slashing through sinew and tissue using some of the adjectives from the powers themselves.
3. I incorporated the "Ming Vase" - and I was very liberal with giving out bonuses and penalties for rolling 20's and 1's. Another thing I did was allow of other effects if someone rolled a 20. For example, if the character was using some power that did not include the moving of the target but rolled a 20 I figured would be fun to move him anyway.
4. I kinda like Skill Challenges - but then again I like subsystems. In a lot of ways the skill challenge is just a formulaic way of doing a reaction roll or some other roll that is already present in B/X.
5. I find the races annoying - I like the fact that B/X is humanocentric. I like the archetypes of race as class. I did not have one human character at the table and while I have read the races sections of the 4E PHBs, I still don't know what archetypes many of the races are suppose to fill.
6. I converted one of Paizo's Pathfinder Society scenarios designed for organized play and it was really easy.
7. The session went faster than I thought. I feel bad about this one. I thought I had lots of material given the feedback I have heard that combats take too long. This was only a 1st level scenario so maybe this changes.
All-in-all it was fun. My biggest complaint was the loss of the archetypes of the various races and classes but I also know that OD&D and Dr. Holmes also say that players should be able to play races other than those presented in the books so maybe I should have a more open mind.
Edit: JB just made a comment on our Online B/X game blog that made me realize the other complaint I have with 4E. In B/X part of the feel and fun is the fact that your character is an average Joe that may become a hero or maybe not. In 4E you start off as a hero and are expected to do heroic things.