I would be much happier if, when a train pulled up, the sign told you what train it was. It's easy to miss the little numbers at the front and when trains are late, you still may want to make sure you catch an express rather than a local.
Pretty much, it'd be the equivalent of BART's station signs telling you which terminal a train is going to, just in CalTrain's case you'd be finding out which stops a train was making.
Also: it would be very handy if the signs would give a heads up about how many bike cars there are going to be. With a little advanced warning about a second bike car, us CalTrain bike riders would happily divvy ourselves up into two parts of the platform.
It would get us all on the train, and everyone underway, a bit faster than the current system of conductors coming out to tell us about a second bike car--usually not until most of us are already loaded, so there's really no time savings for the last few stragglers to huff it halfway down the length of the train.
Of course, none of this has to be done with fancy-schmancy electronic platform signs--actual physical signs hung in the windows of the train would do the trick too!