I was a NASCAR regular from age 5 (in 1975) until the late 80s. Back then, NASCAR was sort of a little secret that we had in the Southeast. The racing was astounding, particularly in the 80s -- close, accessible, good personalties, decent tech. I liked Indy car and F1 too, but the high drama of NASCAR just blew me away. You can't watch the end of the 76 or 79 Daytona 500s and not see why NASCAR is so big today.
Here's what killed it for me though, in no particular order:
1. End of accessiblity/personality. My dad and I used to go to the same hardware store in SC that Cale Yarborough did. You could walk around the pits at Darlington and talk to just about any driver. Richard Petty was just about the nicest guy you could come across, etc. etc. etc. These guys were NOT prima donnas, and not little 5'7" copies of some sponsor friendly cut out. David Pearson, Tim Richmond, Darrell Waltrip, Buddy Baker, Jimmy "Smut" Means (imagine a sponsor today hiring a guy with that nickname), Joe Ruttman, Ron Bouchard, Lake Speed, Dick Brooks, Bobby Allison, Benny Parson, Ironhead (hated him), neil Bonnett, Harry Gant -- all of those guys had personality.
2. Money. Used to be a good Winston Cup car cost maybe 50k, and it was a tech marvel for what it was. Find the mid80s Road and Track where Harry Hyde and Tim Richmond took a (cheated up) Monte Carlo SS and beat the snot out of a Porsche 962 at a high speed test at the big Goodyear test oval. Excessive money/testing/tech kills racing, like in F1, and in Indycar in the 80s, and now NASCAR.
3. The "spec" car. Until the late 80s, I could look out at the track and see Richard Petty in PONTIAC, and Ironfart in a CHEVY, and Harry Gant in a BUICK, and Bill Elliot in a FORD and know what the cars were. It's maybe a small thing, but losing the manufacturer identity to essentialy a spec car I think has hurt the sport.
4. Loss of teh short tracks/small venues. 15 years ago, Nascar started to ditch places like North Wilkesboro and Rockingham for the cookie-cutter 1.5 ovals in Vegas, kansas city, etc. Mistake. The racing on the 1.5 ovals sucks. At Wilkesboro and Rockingham? Awesome.
5. The retirement of the bias ply tire and the rise of aero. Yes, it was old, old tech. But the bias ply tire really allowed the driver to drive the car. In the 80s, you'd sit and watch guys basicaly drive the car sideways through the corners, with smoke off the right rear. Now, the radial, plus the aero crap, means a parade in the low line. The new car has admitedly helped change this some, but still, the fact remains, you CAN'T run the Petty/Gant line up against the wall like you used to. Tires and aero changed this.
6. This Chase BS. I hate it.