All
Trains with sufficient carriages to meet regular demand. Daft always sanction bids with virtually no or little growth and thus the production line is generally shut before the problem can be addressed or new pollution rules mean you can't buy same stock.
Blanket rules for new trains for emission standards are mad. The emissions from trains ought to be compared to those produced by air, sea, car and bus and equivalence sought. So the emissions from one freight loco ought to be compared to say 20 lorries and if currently less then left alone.
Short distance trains
Toilets (at least 2 and easily maintainable) shouldn't be allowed out without a working toilet.
A seat with a cushion not resembling a piece of solid hardwood - only criticism of Class 333 in fairness - otherwise excellent commuter trains.
On medium to long distance trains
Working toilets
Comfortable seats. Nearly all modern long distance stock has poor seats, being far too hard. Its somewhat surprising that train seat designers appear never to have sat in a car seat - perhaps if they had the might come up with something comparable for trains.
Ironically, the LNER (nee Virgin) first class leather seats are unduly wide and somewhat hard causing a normal person to slide around lost without lateral support. The designers appear to have used an over-enthusiastic user of maccyd's as a model. Doubtless there should be a few generously proportioned seats in first class, but these are ridiculous. A reclining mechanism that worked and was maintainable might be of advantage too.
Free wifi
Long distance
Decent hot meals as opposed to disgusting soggy, fatty microwaved bacon sandwiches etc, prefrably cooked in an oven. However, decent Curries, Risottos, Steak pies etc work well even in a microwave.