One advantage that the steam tank gains is that it never has to worry about the steam point system. The steam tank is no longer crippled by a pair of wounds. Additionally, there is no risk of self harm that comes with expending steam points.
This here shows me you don't understand critical elements behind the design of war machines. The Steam Point system is a high risk high reward system with a random element. What you're doing here is making the Steam Tank less random, less unique, and honestly less epic. Warhammer Fantasy is not chess, if you want to minimize the influence of dice play something else.
This here shows me that you don't understand this guy.
Let's try and be a little more constructive, my good man.
::adjusts monocle:: Ahem, old bean, let me take a stab at this.
::clears throat in distinctly British tone, the sort of throat clearing that typically comes with a thick handlebar moustache and a few too many years in Her Majesty's service:: You see, the Steam Tank rules, as currently written, are indeed complex and have their drawbacks. I for one agree with this.
You state that the tank loses a great deal of its effectiveness as it loses wounds, and to me this is not a drawback, this is to be expected with something as complicated as a steam tank. Tanks are complicated devices, and it seems natural and fitting to me that it should become less powerful as the armour is punctured, the vavles rattled, gaskets blown loose, and crew wounded by shrapnel and miscellaneous pointy bits. One could argue that perhaps the tank is overly punished for wounds, but tweaking this system is, for me, more in line than discarding it for one where the tank can be tottering about on one cracked wheel (wounds wise) and be just as effective as full strength. I know, I know, monsters are in the same boat, but we don't have dragons in our world for reliable comparisons. Perhaps mythical beasts are just that professional to tough it out under extreme duress.
Secondly, you remark that with the current rules, the tank just "sits there like a big lump of metal". Well, that's what tanks do, old boy. They're not particularly proactive about fending off sword-wielding maniacs. The enemy should get automatic hits, because it is the general nature of tanks to apathetically sit there and let people stab them when they are close enough to do so. I agree that having a whole turn with no combat response is absurd, and needs to be fixed. Why can't the engineer pop out of his hole and shoot in response? Perhaps the tank gets 1d6 S4 Armour Piercing shots every enemy turn in retaliation, to represent the crew firing handguns (or pistols, tweak the stats accordingly) out of strategically placed portholes, or maybe a free blast from the steam cannon each turn (doubtful, this seems too powerful but it's a thought). The variable factor comes from not necessarily being able to shoot at every porthole with equal ability. I think this works better than making it a chariot type device with a weapon skill; chariots have mounts to pull them, crews to defend them, and sometimes even scythed wheels. All the tank has is a crew contained within its mammoth depths, and I for one think if it's going to defend itself, that crew should be its only option.
Again, I don't think tanks should be able to make pursuit or overrun moves in the same fashion as a chariot. Chariots, you can simply whip your steeds and drive after the foe. Tanks need to be put into gear, pressure built up, etc, and all that comes after your heavily blinded and deafened to the outside world crew realizes the enemy is in full flight.
As for the steam points system, it is indeed complex, and I think unfairly punishes the tank for wounds (it does lose much effectiveness very quickly). However, I feel that these rules more closely fit the image, and imagined gameplay, of such a device than yours.
That is my respectful and reasoned disagreement. Care for a spot of Port, old chap?