I'm not a fan of the seating on the 700s, they are too small for my 6'3" body to fit, especially lack of leg room and far too close together to be able to sit comfortably.
However, the real problem is they are becoming a hotbed of anti-social behaviour, and this is as much to do with design as operating practices. Last week I did 6 journeys on them over 2 days - average journey length was around 40 minutes from a supposedly not so bad bit of Surrey.
On 4 journeys Beggers/Tissue Sellers taking advantage of the open layout to pass their charms and demands onto very uncomfortable passengers. It is both the design of the trains and lack of onboard staff that combine to make these trains ideal for the beggar gangs to set up and operate on. The open through layout is good for sucking in passengers but the openness of the seating means it is easy for beggars to get to those sitting down. As I head into my senior years these approaches feel more and more threatening. After many years of using late night train services regularly I would now rather drive into London at night than use a Thameslink train - although to be fair that is also partially about the lack of confidence in the service running.
On both the other two trains I was sitting in rear first class (declassified), in the first train a passenger with a dog sat opposite me and he encouraged his dog to sit in the seat, in the second a family took up a nearby table for four and set about making the most foul-smelling sandwiches I have encountered which again the open design meant the smell permeated the whole carriage. In both cases no Guard or OBS to ask them to refrain.
Using the train has become far more unpleasant since the 700s have appeared - it may be about general behaviour, but I do think the trains are partially at fault as when we had the occasional 319 and mostly 377's to London Bridge these problems almost never happened.