We tried this out last night, along with the Gang, Shortbow and (new) Dashing rules. All seemed to work nicely. The game was a treasure hunt pitting night goblins and a bugbear against lizardmen with a dinosaur-rider (a GW cold one rider ridden by a lance-wielding lizardman). The profile for the dinosaur-rider was Q3, C4, Mounted, Trample, Dashing, Long Move.
This gave a pretty fearsome heavy-cavalry profile, but it "felt" right. In the past, we've given the dinosaur-rider Savage, which, in one short-lived game, caused him to rout an entire Orcish warband on his own, with a single kill. The Trampling iteration worked well, though.
There was an interesting interaction of the Trample and Short Bow traits and reactions; my goblin hero (only C2 but with Good Shot and Short Bow) crept close to the rider, hoping to take him out with an aimed shot at Short range (which would be C4 vs C3 in his favour). But a failure and reaction led to a charge by the rider; Dashing gave him a free attack, which led to a knockdown, and the poor goblin was then trampled at Q7 vs Q2. Harsh, but it looked right on the table, given the bulk and fearsomeness of the reptiles, and the rider was a cool 100 or so points that could have been taken out by the archer, especially as the bugbear was waiting close by to provide the coup de grace; he, though, routed off the table at the sight of the goblin's gruesome demise.
So, the Short Bow bonus for Short range gives a great incentive for archers to get close - but with the reactions, that's a very dangerous game to play. Trample really gives heavy warhorses and monstrous steeds some extra "oomph" - I could see it working terrifically well for a black knight on a destrier or the like. The combination with Dashing is particularly potent, but again, I think that works well with exceptionally powerful cavalry; a goblin charged by a dinosaur-rider should probably come a sticky end. Finally, reactions make Dashing models (with the new version of the trait) particularly dangerous. This really helps to break up any sense of "I go, you go".