Vehicles are generally absurdly powerful in the W40k universe, so that will be a fine line to walk while they design the damage output and durability.
With that in mind, I doubt they'll put vehicles out there in the wild (a non-pvp area) due to how difficult it would be for just a few players to take out a vehicle. Plus the issue of how suddenly a battle can be ended by destroying a few vehicles. I doubt many people would be happy with a PvP system where you target the vehicles first or you die. That isn't really PvP. :/
Including Melta Bombs would solve a bunch of potential issues, but would they require materials? A specific profession? How much would they cost? Here's hoping they're near free or are dropped as common random loot, because after all, they only have a niche use.
My biggest worry as far as vehicles goes is having another Strand of the Ancients from World of Warcraft. It involves using vehicles to destroy several gates in order to get to a relic behind a final gate; fastest time to the relic wins. The problem is, there wasn't much PvP and a few classes were infinitely better at it than others. Yes, some classes will always have an advantage over others, but if a class can single-handed stop an entire team's progression in its tracks, there is an issue.
I'm not too fond of mounts, so customizing those wouldn't be in my interest. They'll definitely be in-game, but the idea of summoning a living animal or a several-hundred pound bike from thin air is a tad odd. Carrying a few thousand pounds of gear and junk is odd too, but that disbelief can be suspended. It just seems that we can get a bit more creative than the stereotypical MMO style of requiring a mount to play at any reasonable pace, especially when we have drop pods and various ships, etc.
I'd prefer decals and customizations on my actual character, such as tattoos, especially if we can have user-submitted tattoos. Let me shove a popular player's head on me pointy stik please. Weapon and armor customization would be awesome too.
As for the physics of it, crashing would be nice, but it isn't as important as the vehicles being destructible. I'd love to be able to aim at certain parts of the vehicle to disable it (how else would orkz get a looted tank) or perhaps jump in and fight the players for control.
