In response to the OP I'll give just some of my reasons.
That AWFUL colour. Described as "woad-like" blue in an article I read. Tatty green diesels having freshly painted enormous yellow ends/noses. The fact that the upstart diesels didn't even have to carry a "D" to make them recognise the senior motive power.
The stations and infrastructure were at their shabbiest and most unwelcoming. There wasn't anything positive going on, just more decline and closures.
Mk II coaches and B4 bogies. Nuff said. Goods trains started to lose their brake vans and look just wrong.
Did I mention the colour? Oh, yes, the fact that there was no more freedom given to the Regions. Apart from the best liveries ever of green loco's and maroon coaches we lost the distinctive green coaches of the Southern, the (G)WR brown and cream, the red loco's-diesel and steam, and some of the really nice schemes applied to some diesels (Deltics and Brush 4's to name just two)
I still have my Ian Allan ABC of BR steam loco's for 1967 IIRC, and a sad thin little volume it is . I've probably got more steamers than they did!
And finally, the colour. The final degrading insult of all in painting the VoR trains. I suppose they felt it was important to make them look as though they belonged to BR to distinguish them from the amateur efforts elsewhere.
Ed