I have to say that it has been a very long time since I have played a tank as frustrating as the D2. If it is one overwhelming flaw it is its speed. It just takes so long at times to get to the action often there seems little for one to do.
The slow speed comes about, in part at least, because the tank is well armoured for a Tier III vehicle – and against Tier III vehicles that armour can be effective. Not so much against the tank destroyers of course, but against the regular tanks of that tier. At Tier IV however the armour loses its efficacy. It will still bounce some shots, especially from un-upgraded weaponry, but pretty much everything should be able to puncture through reliably, the odd bad shot aside. Once you reach Tier V the armour is, more or less, meaningless.
If the armour is fairly good for a Tier III, the firepower is horrible. The top gun will suffice at Tier III, but low penetration rolls will bounce, and effective angling can create problems. At Tier IV the effectiveness of the gun without premium becomes very situation dependent. Trying to pierce a Matilda, for example, will be an exercise in frustration. Once again however the real weakness is at Tier V. The regular ammo has an average penetration of 66mm, which will have some success on the right targets, but far from reliably. The premium ammo has an average penetration of 98mm. This will have more success, but will still be unreliable against the heavies.
Of course, the gun weakness is a problem many Tier III tanks have at Tier V, but most of them have more mobility. The horsepower/ton is only just above 9, and the acceleration is poor. The track traverse is average, but nothing special. The turret traverse is poor. Apart from taking a long time to get anywhere, this means the tank can be easily outmanoeuvred, and once it gets into trouble it usually dies. To some degree though this weakness is both expected and something one can plan for. Ultimately the biggest weakness at all remains how long it can take to get to the action.
My own record in the D2 is fairly successful – 19 matches, 11 matches, and 19 kills, and a 1st Class Tanker medal. I played a few more games than I otherwise might have trying to get the Ace, but I think I am going to forget about that. Slow speed makes one much less able to adapt to the randomness of teams. Sometimes slow speed can come with other compensations, such as firepower, but there is no sufficient compensation in the D2.