I'm going to argue that it was doctrine and tactics rather than the weapon that made it so effective.

First a note on theoretical versus actual rates of fire. On the battlefield weapons will never achieve anything near their theoretical top rate of fire due to a variety of issues- ammunition supply, suppression effect, accuracy and fire discipline. untrained soldiers tend to fire widely, in an uncontrolled manner wasting ammunition, whereas trained soldier will observe and fire aimed shots or short bursts in a controlled manner.

Generally most infantry soldiers equipped with bolt action rifles fired 5 round per minute deliberate and 8-12 rapid (the British mad minute of 15 rounds being the exception, based upon years of training and exceptional fire discipline). While the British Bren team was expected to fire 30 round per minute on deliberate, 50 rapid, in comparison MG34-42 gunner were trained to fire at 50-100 round per minute. The reasons are to due with the weight of the weapons (in terms of KG, not bullet size) resulting in a weapon that is more controllable at higher rates of fire and the advantages of Belt vs magazine fed systems. as can be seen, theoretical top rate of fire are less relevant than they might at first appear. It is also evident that there is already a firepower unbalance even before tactics are considered, British infantry had been armed in the 20s to fight small wars on the north west frontier of India against riflemen and snipers, while the German infantry was armed for continental war. It was the conceptual state of the German army that was to result in a devastating synergy of tactics and material.

Of primary importance is to understand that the German regimental organisation was based upon a limited German industrial base (in comparison to their enemies), the terms of the Versailles treaty forbidding artillery and tanks and an emphasis upon integrating first world war storm troop tactics into the German army, via section tactics and battalion heavy weapons- deploying infantry guns, light and medium machine guns and light, medium and heavy mortars as integral weapons to the unit.

This resulted in a huge number of MG34s of MG42s deployed as both light and medium machine guns, either as platoon, company or battalion assets, and consequently a German infantry regiment has a far greater amount of integral firepower than an equivalent British commonwealth brigade or American regiment, who tended to concentrate their medium machine guns at battalion or regimental level, or in alterative units. additionally weapons were often either unique to the Germans (infantry guns) qualitative better (LMGs/MMGS, heavy mortars) or offered at greater scales granting an quantitative advantage (possibly medium mortars- open to debate). Note that the Germans placed little emphasis upon artillery support while American and British forces were to increasing do so as it effectively exploited their material advantages and compensated for inferior infantry weaponry.

Secondly is the issue of how this weapons were deployed at the section/squad level. American troops were generally handicapped by a lack of effective LMGs, with most infantry squads armed with a B.A.R., trained to deliver automatic rifle fire rather than as a true LMG, though they seem to have secured additional SMGs for themselves and possessed larger squads of 12 men. British infantry sections or 8-10 men were slightly better prepared through the emphasis upon utilising the Bren gun as a proper LMG. This advantage how ever was negated by poor tactics, as the entire section was expected to 'pepper pot' forwards in individual or pair fire and manoeuvre. the result was that the Bren gun, which represented the majority of the section firepower was continuously redeploying, limited the moments when it was able exploit its higher rate of fire, the emphasis on forward movement reducing firepower. Additionally their single SMG and bolt action rifles limited their firepower in close quarters battle in comparison to the automatic riflemen of the US or German SMGs. the result of the moving Bren gun vs their stationary opponents effectively gets us into a situation where the Bren would be firing at a deliberate rate against the rapid rate of the MG34 or 42.

The German infantry squad was far bigger, better armed and effectively deployed than their allied opponents. A 13 man squad was broken down into a LMG team, a rifle team and a assault team with SMGS, and was fully capable of fire and manoeuvre. First the LMG team would be placed in an optimal position by the section commander. Once emplaced, it would be static and produce an overwhelming amount of firepower under which the rest of the infantry squad would advance. The rifle team would be dropped off next to form a point of fire, their aim was to provide accurate rifle fire from a alternative position, the result was that the enemy position was now under fire from two separate positions and quite pinned down. in this state, they would be vulnerable to a sudden assault by the assault team wielding grenades and SMGs

references

English, J.A. and Gudmundsson, B.I., On Infantry, (Westport, Praeger, 1994), pp42-45

French, D., Raising Churchill's Army, (Oxford, Oxford University press, 2000)pp38-40

Place, T.H., Military training in the british army, 1940-44 (Abingdon, Routledge, 2000)p, 69-73

idwell S., and Graham D., Fire-Power (Barnsley, Pen and Sword Books ltd., 1982)p, 193-195, 206-208,

The MG42 was not unique in terms of rate of fire. In fact, the Soviets had the highest RPM gun of the war in the form of the ShKAS (although it was primarily used on vehicles, since it chewed through ammo at such an insane rate that carrying around enough to feed it would be difficult,) with a RoF of 1,800 rounds per minute (and one limited variant which apparently went up to 3,000 rounds per minute.)

(EDIT: A minor correction, the Germans DID produce a gun with a higher RoF, but they did so by basically strapping two MG-81s together for a combined rate of fire of 3,200 RPM at max.)

The MG-42 was effective because it was simple and easy to mass produce. Compared to the earlier MG-34 and the ShKAS, both of which were relatively complicated to produce, the MG-42 was made with lots of stamped parts, allowing it to be produced in 2/3 the time of the MG-34.

Basically, in terms of performance, the MG-42 wasn't anything special. Even the humble US browning M2 had a variant that could match its rate of fire. IIRC Brazil still uses it in its trainer planes.