As I have said before, I find ticket gates/barriers to be the single most annoying aspect of rail travel in the 21st Century.
Process: This is another point where we the paying public are being PROCESSED and do not like it. Sheep going for dipping or shearing are treated in a more pleasant manner than the way some gatelines are arranged.
Rejects: The gates are AWFUL at rejecting perfectly valid tickets. Particularly when the user is starting short or making a break of journey but at other times as well, particularly shoulder peak (ie when the first off peak train from Reading is 0934 and they will not allow an off peak ticker through the gates until 0931). Another instance is Rovers - I had an East midlands Rover a few years back and it wouldn't work the gate at Derby (2 minutes and 20 feet away from when and where I bought it) or at Nottingham or Lincoln.
Retention: There are LOTS of reasons why a passenger may wish to retain their ticket at the end of their journey, and the retention programming turns this process into a faff. Reasons could be delay repay, expenses, personal records, sentimental or more. This ends up costing me more money as whenever I am travelling to a gated station I end up buying a ticket to a further point so that it will not be retained at my destination. Reading to Oxford this becomes Earley to Hanborough and so on.
Delays: The whole process is fine if passengers are not carrying anything. Add a suitcase, camera bag, laptop bag, paper bag with lunch in it and a steaming hot cup of tea and you have a recipe for disaster. I have in the past had aforementioned steaming hot cup of tea knocked out of my hand when using a barrier, and the outcome was not pleasant!
I would advocate removal of barriers from all major stations, particularly those on InterCity routes, and especially London terminal platform such as Kings Cross 0-8 and Paddington 2-5 and 10-11. A higher penalty fare and random "heavy checks" should support the tackling fare evasion piece, and for security well this is a complete misnomer which does not stand up at all.