Personally I regard it as:-
Poor provision for those at each end of the academic ability spectrum, i.e. those at the bottom who really struggle where the year by year school system is simply too fast for them and those at the opposite end who aren't stretched and get bored by the slow pace of the year by year system.
Lack of choice due to the very limited number of subjects available, very limited number of sports/games options available, limited choices as to exam options (must choose one subject in each block so, say, have to do a language and can't do two humanities kind of thing). Basically, the scope and breadth of their education is limited as to what others think is best for them rather than what they want to do.
But yes, the exam system too. I far preferred the old system of O levels alongside CSEs rather than the one size fits all system of the combined GCSEs.
And finally, the complete lack/absence of technical/manual skills on offer at the typical comp. Woodwork and metalwork "teaching" really has barely changed in 40 years and is still looked down upon, not taken seriously etc. Today, it's as true as it was 40 years that the only pupils who do well "making things" are those who have a father/grandfather/uncle with a workshop in their garden shed, which is where the real creativity and skills are learned, i.e. actually doing it, being inventive, etc. The school idea of spending an entire term making a wooden fish followed by a term making a metal trowel is woefully inadequate in nurturing interests/skills in manual trades work. For GCSE the majority of the marks available are theoretical based on the research, drawings, exploration of materials, etc., so basically still academic so not good for less academic pupils who'd rather spent two years actually making stuff rather than creating a written project folder!