I have a few problems with the 700's. Aside from the already mentioned window seats designed for people with one leg, the passenger information system on these trains is awful. If the wrong software version is loaded onto the train then the SDO information is messed up, scrolling text is very buggy (often gets cut up), it often cuts out/gets lost in the core owing to the short distances between stations and most annoyingly it doesn't keep track of changes to calling patterns/destination which happens very often on GTR (and most drivers seem to have a phobia of picking up the PA handset). Additionally, the display housings seem to be very ineffective at protecting the LCD displays, many of which are now damaged to varying degrees.
They are also very dirty, with some areas seemingly not cleaned since new (like the bits between the seat and the wall/other seats). Oh, and some of the seat bases in standard are collapsing now, that is if they are not loose because maintenance forgot to screw them back in. These aren't necessarily design issues though.
One more gripe, the doors. Great for commuter services in the core, not so great on 100mph lines passing trains going the other way at the same speed. The problem is that the doors have a bit of play in them, if you are on a full and standing train and are pinned against the doors you need to make sure your head isn't near them as getting hit by the doors jumping as a result of a passing train is not pleasant.
They are great inner suburban/metro trains with their high capacity and wide doorways, but they really aren't suited for medium to long distance commuter services.