Almost all of the problems are down to the way that Virgin had them built, but basically its as follows:
i) many seats do not line up with the windows (indeed at least two on each coach have a view of just the pillar)
ii) the seats aren't that comfortable anyway, at least when compared with those on the east coast or those that used to be in service during BR
iii) the legroom in standard is, with the exception of four seats per coach, substandard
iv) the smell of the toilets that still occasionally pervades them
v) Virgin still run them in service, even when important parts of the system (like the air conditioning, on a day when its 30 degrees and the train is full) are broken
vi) the catering facilities are woeful, especially in first class
I have to use them to get home, but whenever an alternative exists (like the two blissful years when the Wrexham - London direct train was running) it was an easy decision to not go on their awful service.