what made the Bismarck so great?

Bismarck was a large fast battleship. As such, it was a powerful warship. The problem it caused for the British was that it was faster than their old battleships, including their newest old battleships (Nelson and Rodney) and refitted older battleships (e.g., Warspite, a Queen Elizabeth class battleship built before WWI and refitted in 1937). When Bismarck was commissioned in August 1940, the only ship the British had with the speed and armament to match her was HMS Hood, a WWI-era battlecruiser. Hood was similarly fast, and like Bismarck, carried 8 15" guns. Hood was less protected than Bismarck - her armour was generally thinner (as one might expect from a battlecruiser vs a battleship). A few months later, King George V was commissioned, and Bismarck faced another formidable opponent. King George V was a good match for Bismarck in a fight, with 10 14" guns and thicker armour than Bismarck, although she was a few knots slower. Another few months later, Prince of Wales (also a KGV class) was commissioned.

This left the British with the challenge of protecting their convoys if Bismarck was used as a commerce raider. Escorts other than battleships were vulnerable to Bismarck. The old slower battleships (e.g., Rodney and Nelson, the 5 QE-class battleships, and the 5 R-class battleships) could be used to escort convoys, but Bismarck had the speed to avoid them. Using them as convoy escorts would make them unavailable for other purposes. Thus, when Bismarck sailed toward the Atlantic, the British were strongly motivated to stop her. Prince of Wales and Hood caught Bismarck and the accompanying Prinz Eugen. The battle did not go as well as the British had hoped - Hood was hit by both German ships, and her magazines exploded, with only three surviving crewmen picked up afterwards. Prince of Wales had trouble with malfunctioning guns and turrets (she was newly commissioned and clearly not debugged yet), and was damaged by Bismarck, and had to withdraw. She did hit and damage Bismarck, causing sufficient damage for Bismarck's raiding mission to be aborted. This battle (the Battle of the Denmark Strait) shows that while Bismarck was a powerful battleship, she wasn't a super-ship - Prince of Wales and Bismarck caused enough damage to each other to end their missions.

Bismarck's next move was to head for home, for needed repairs. Her problem was that she was a very high priority target for the British, and without the kind of luck that had let Bismarck and Prinz Eugen sink Hood, could not expect to successfully take on multiple British battleships. The British problem was trying to catch Bismarck using slower battleships (and the equal-speed King George V and Prince of Wales - Bismarck had been slowed somewhat by the damage inflicted by Prince of Wales, but could still make 27 or 28 knots (normal top speed for the KGV class)). With only two battleships that could match Bismarck's speed, the British task wasn't easier. However, a carrier-based air strike found and hit Bismarck, and the damage reduced her speed to 16 knots, soon raised to about 20 knots. Still, the British had to keep track of Bismarck, and re-locate her when they lost track, and get battleships into gun range of her. This takes time, and that time meant that the British ships hunting Bismarck ran low of fuel.

Might Bismarck escape? Alas for her, a second airstrike caused further damage, leaving her unmanoeuvrable, only able to steam in circles, and the British ships caught her. This force consisted of two battleships (Rodney and King George V), 2 heavy cruisers, 1 light cruiser, 7 destroyers and 1 aircraft carrier. Yes, this was a large force (13 ships), but in a gun battle, it would be the two battleships that counted the most - they had the guns to do serious damage to Bismarck, while having the armour to have a chance against Bismarck's return fire. Rodney started the shooting, at 8:47am, and was joined in action by King George V. At 9:02, Rodney hit Bismarck's bridge, killing most of the senior officers. The same salvo knocked out Bismarck's main fire control director. Her main fire control director took over, but was knocked out 3 salvos later, and Bismarck's fire was now essentially uncoordinated. The heavy cruisers Norfolk and Dorsetshire took advantage of this to move into range where their smaller main guns could contribute. By 9:31am, all of Bismarck's main guns were out of action - she had returned fire with her main guns before they were knocked out, but had failed to score any hits. Rodney moved into close range - she was still almost 3km away, but that was effectively point-blank range for her main guns, and she proceeded to demolish Bismarck.

In summary, the main British problem was catching Bismarck with slower, or at best, equal-speed ships. The two carrier-based air strikes let them catch Bismarck. Once caught, 2 British battleship quickly overwhelmed her and left her a floating wreck.

Bismarck wasn't yet sunk, but she was out of action. Battleship guns are fine weapons for putting battleships out of action, but poor weapons for sinking battleships. Guns tend to hit the part of the enemy ship that is above water, while you want to put holes in the underwater part to sink her. Torpedoes (whether ship-launched or air-launched) can be more effective, but battleships were armoured to resist torpedo damage. The difficulty in actually sinking Bismarck was to be expected. However, it wasn't necessary to sink her - she was defeated and out of action. Still, they wanted to sink her, and hit her with two cruiser-launched torpedoes. By this time, Bismarck was listing so badly that part of her deck was under water. Even without the torpedoes (or scuttling), Bismarck was sinking. With the torpedoes (and/or scuttling charges), she sank faster.

In principle, after the air strike damage to Bismarck's speed and steering, Rodney could have crippled and sunk her, acting alone. Using two battleships rather than one was prudent, and Rodney was accompanied by King George V, rather than acting alone. The other ships contributed to the sinking, but their main contribution was those two torpedo hits when Bismarck was already doomed to sink from the gun damage alone.

A suitable battle for comparison, also with battleship vs battleship gun action, is the Battle of Surigao Strait (25 October 1944). Two Japanese battleships (and accompanying cruisers etc) were ambushed by 6 US battleships (and accompanying cruisers etc). One of the Japanese battleships, Fusō, was sunk by destroyer-launched torpedoes early in the battle, and the other, Yamashiro, was crippled by gunfire from the US battleships, and then sunk by torpedoes. Similarly, in 1942, the Japanese battleship Kirishima proved resistant to sinking. The US battleship Washington snuck up on her in the dark, and opened fire at the devastatingly short range of about 5km, and hit her with at least 9 main gun shells, leaving her crippled. A Japanese cruiser attempted to tow her to safety, but gave up and took the crew off the ship instead. Crippled and abandoned, Kirishima capsized and sank about 3 hours later (and therefore stayed afloat for longer after the shooting stopped than Bismarck).