Don't know what you're talking about. Just about all my perfections came from that thing!
It won't make you invincible, you've just gotta use it correctly. Although you might think otherwise the Mantis is more suited in the back of the line. Take some pot-shots at your enemy to weaken the shields from behind a rock or wall and move back when overheated/reloading, some well placed rockets will destroy vehicles and infantry alike.
But for god sakes don't let the enemy come too close to you!
Now for what I think it should definetly return. It was a tank like vehicle with just a tad more mobility and controllability.
But when it does return it needs a few minor tweaks in my opinion. Some things that sound like common sense to me and was quite surprised about that it wasn't integrated.
Like manually reloading your rockets and main cannon. It feels a bit wierd that I'm obligated to fire ALL my rockets in order for it to reload, and when your main cannon is about to overheat I'd like to manually vent them when there's nothing nearby. Another thing that was a bit wierd to me was that you could charge the rockets when there was only one left. What's the use in that?
The second thing is that the Manitis feels kinda clucky. The stomp to get rid of nearby infy is almost obsolete due to it only having a small area of effect in front of the Mantis. And when you do manage to stomp someone, you'll be exposed to any other threats because you're forced to stand still foor a little too long. In my opinion the radius of the stomp should be increased and the cool down time should be shortened if not removed completely.
Another thing that went a bit too slow was getting in. A lot of people avoid getting in the Mantis on Exile because you'll be a sitting duck to snipers, rockets and Spartan Lasers while you're getting in. This time should be shortened.
My last point, and this one is about all vehicles, is that it feels weak. Most vehicles, including the Mantis, feel like they're made out od paper maché. In the previous Halo's when you saw a vehicle your first reacion wouldn't be to shoot it with your BR or AR as it would do little to no damage. The wierd thing in Halo 4 is that almost all weapons to tons of damage to vehicles. Not to mention being able to spawn with a weapon that stuns them for 5 seconds...
Don't get me wrong, the Mantis is a powerful vehicle in Halo 4. Just with these changes I feel it could have much more potential!