For me its not so much the narrowness, although that's definitely a big part of it. For me, its more the closeness. Once you're in the seat and you have it adjusted so you can reach the pedals and everything else you need to reach, everything is... close. Its all right there. There is no need to reach out for anything, it's all right there. The panel feels right in front of you compared to the Cessna's and Piper's you might be used to flying.
That's actually what I like about it! I did most of my instrument rating in an Archer, and I would end up with a backache after every flight because, at 6'4", I can't reach the panel in an Archer without leaning my entire upper body forward. In the Mooney, everything is accessible by arm movements only. It's a GREAT airplane for tall people, like I said before, and even not-so-tall people. I have my seat on the farthest back setting. The only person I know who used the available pedal extensions in a Mooney was 5 foot nothing.
It was the getting to the left seat in the M20J that wasn't so great, but I only flew it a few times, so I probably just didn't figure out how to do it well.
At 6'4" and 315#, I've flown a lot of airplanes and it's *always* about how to get in. Heck, I've flown a J-3 Cub. In the Mooney, I finally figured out that it's easiest to just face the tail of the plane standing on the wing walk, drop down so that my right knee is on the right seat, and then just roll right into the left seat. I kinda laugh at the "Ultra! Two doors!" that Mooney was hyping more recently because even if I had an Ultra I'd get in from the right side, I find it way easier to get into the far seat than the near one now that I know how to do it.
There have been a few I couldn't fly at all:
* Columbia 300. Sitting in the left seat, I was looking at the door frame.
* Diamond DA-20. Same sort of thing, I'd have had to cut off about the top 5 inches of my head to fit under the canopy. The DA-40 fits me fine, though.
* Globe Swifts with the "152 seat" option that lets you move the seats also makes the seats too high for me.
The 152 is also tight enough that, while I can get in, it probably wouldn't be particularly safe for me to fly with the geometry of the controls and seats.
The trick on the J-3 (or Super Cub or Husky) is to sit at the far back end of the door, get your legs in through the front of the door, and pull yourself forward into the seat using the bars above the glareshield.