On trains which have refreshments trolleys, space for storing them and for the attendant to sit should be provided. Often they're left clogging up one of the doors which is a bugger for wheelchair users or very busy services i.e. Stanstead to Birmingham.
Instead of us buying tickets and then maybe missing a train, getting the wrong one, or whatever, we should get a pass which we sign on at one station and sign off at another. We'd then pay for this pass at the ticket office and it would be charge us whatever that journey was worth.
Reserved seats need to be more smartly thought through. If somebody has reserved a seat, they should have to inform staff if they are not going to use it. I've seen people reserve a whole table, but it not be in the right place so they've gone and sat at another table closer to the onboard shop or whatever; which leaves four seats which can't be used by anybody!
Longer trains. There are so many, practically all services, from Birmingham which are over capacity.
Less "smart looking" train layouts (see: Pendelino airline seats, countoured walls taking up space, etc) and more spacious layouts instead. Trains these days are too fashion-based: because every new train that comes out has a different style, trains are able to "look old" (take 150s for example).
A standard, good-looking livery. We don't need to know which TOC is running a route when it's the only option for that route, and some liveries are hideous!