My controversial opinion:
Seat reservations should be abolished, or charged at a level that discourages people from booking them speculatively or for short hops (e.g. £5-£10 a leg) and limited to maybe 1/4 of the train's seating capacity.
They seem to cause increased dwell times, arguments between passengers and conflicts in passenger flow as those walking down a platform/carriage to find their booked carriage/seats have to try and pass others coming the other way!
Boarding would be so much smoother if passengers knew they could just jump onto any coach and into the first free seats they see, clearing the aisle.
My local station is served by XC (lots of reservations) and Chiltern (no reservations) and even allowing for the differing door positions of a Voyager vs 168, the Chiltern trains are much quicker and easier for everyone to get boarded and sat down!
Because reservations are free, even for the shortest journeys, people also often book reservations they don't need. This leads to people being unsure if they can grab a seat that looks like a 'no-show', plus blocks the seat from being reserved for a longer distance journey where it might be worthwhile.
The only thing I would do is allow free seat reservation for any holders of Senior/Disabled railcards, or any booking that includes both an adult and child ticket (to give families a chance of sitting together).