update May 18
Note: Below I failed completely to note that Misrata is a major port, serving a large surrounding area. The port being in rebel hands and the surroundings in government control is a central gripe behind the "tribal threat" of late April.
Misrata (alt: Misurata, Arabic: مصراتة) is Libya’s third largest city (after Tripoli and Benghazi), boasting in peacetime about 550,000 people. Considered Libya’s commercial capital, it lies in the west of the country about 130 miles east of the capitol Tripoli.  With the de facto rebel capitol Benghazi itself no longer in immediate danger, Misrata has become the new focal point of the Libyan civil war. To hear mainstream news accounts, this would seem to be from the humanitarian crisis there. However, the real reasons behind the West's desperation to "save" Misrata are certainly more strategic than altruistic.
A Crisis of Disputed Size in the Western Holdout
It was among the amazing number of Libyan cities, east and west, that fell to rebel control within just a few days of the revolt’s start on February 17. This flash of activity was much more violent and pre-planned than the world public realizes, but that was needed to seed the impression that the whole country had “voted” by popular action to secede from the capitol.
After the initial shock of this unprecedented mutiny, the loyalists in the army and within the “liberated” cities re-grouped with an early-March roll-back. In general, rebel support was too weak to last in the west, and caved easily, and by the 19th rebel control was limited to their de facto capitol Benghazi and points eastward. The only exception to this rule was vital and sizable Misrata, then and for the last month the only western city even partially held by rebels.
With its switched-off electricity and water, sometimes severed supply lines, and the flight of thousands unwilling to fight, the city has become much less vibrant, to the say the least. On top of that, there's the siege: neighborhood-scale fighting, tanks, snipers, and mortars and rockets launched from afar. It's been described by rebels as a living hell, with "unimaginable carnage," hospitals overflowing, bodies piling up uncounted in the streets. Rebel pleas always cite among those killed indiscriminately women, children, and the elderly, in every single explosion. It's been said Gaddafi is flattening the city, strangling it, and intends to slaughter every person in it.
However on April 10, Human Rights Watch released information on Misrata showing something less inhuman than all that:
According to Dr. Muhammad el-Fortia, who works at Misrata Hospital, medical facilities have recorded 257 people killed and 949 wounded and hospitalized since February 19, 2011. The wounded include 22 women and eight children, he said. Dr. Alan J. Kuperman, a respected scholar of humanitarian interventions, cited this as evidence that "Moammar Khadafy is not deliberately massacring civilians but rather narrowly targeting the armed rebels who fight against his government" ("narrowly" being relative under the circumstances). Since the fighting there started nearly two months earlier, he finds from HRW's numbers "of the 949 wounded, only 22 — less than 3 percent — are women. If Khadafy were indiscriminately targeting civilians, women would comprise about half the casualties." 
In the roughly 45 days since the seige began, 257 is only about six deaths per day, on average. That isn't likely a complete number, but no more likely to be very far off. In reality it looks less like a genocidal massacre than the six weeks of low-level but NATO-prolonged urban warfare it is. Even presuming a gross margin of unreported deaths, 400 or even 500 dead is really not that high - at most about 0.1% of the population. If the government were trying to kill "as many people as possible," with this much time to have done it, they are failing badly.
The Key to the Whole of Libya
Misrata’s fate has become second only to Benghazi’s. But as with the capitol, the motivation to maintain the third city is not to prevent a slaughter of innocents, but for two interlocking geo-strategic reasons.
One reason is, essentially, the city's location. Benghazi holds the key to Cyrenaica (the region from there eastward), which has remained in rebel hands since the end of February. But as the western outpost, Misrata holds a partial key to the whole nation. It's the last geographic bastion of the illusion that this is a nationwide popular uprising, as opposed to an East-West civil war. The latter, but not the former, could be settled with partition, an answer that makes sense now (any other option will leave substantial numbers of people under the rule of now-hated enemies and "traitors"). But the rebels and their western sponsors want to take the whole country, and so both are quite intent on keeping Misrata in rebel hands.
The other reason to the same effect is the city's size. Gaddafi is fully in charge of only one of the tree top cities (Tripoli), with #2 undeniably lost and #3 contested. As a piece in Bloomberg Businesswek noted yesterday:
And finally, a more immediate consideration, again location-based - when and if the rebel are ever able to attack Tripoli itself, it would be from Misrata, not Benghazi. After the putative pullout of April 23:Rebel control of Misrata would leave Qaddafi in charge of one major city, Tripoli, W. Andrew Terrill, a research professor of national security affairs at the Strategic Studies Institute of the U.S. Army War College in Carlisle, Pennsylvania, said in an interview. “If the rebels have Benghazi and Misrata, they’re looking like a viable force and a legitimate government” and it could make a stronger case for other nations to join France, Italy and Qatar in recognizing them, Terrill said. 
Colonel Ahmed Omar Bani, the rebel spokesman, dismissed reports of progress in Misrata."It is a disaster there," he said. "Kadhafi is not going anywhere. Misrata is the key to Tripoli. If he lets go of Misrata, he will let go of Tripoli. He is not crazy enough to do that."http://www.dailymail.co.uk/news/article-1381068/Hundreds-dead-bodies-streets-city-reduced-rubble-Full-horror-Misrata-siege-finally-emerges.html
Clearly something extraordinary is driving the rebels of Misrata to stay in that "hell" with their families, to reject each offer of truce or negotiation, and maintain the city as both a war zone and a buffer against legitimate government counter-attack. Any attempt to re-impose order with armaments is deemed unacceptable, since the fighters have situated themselves in a city stuffed with civilians. That is, they're using themselves and their families, in a sense, as human shields.
And when Gaddafi's forces aren't willing to deliver horrible enough atrocities, the rebels or someone sympathetic will fake the most incendiary crimes for them. Children targeted by snipers in Misrata widely claimed, but only "proven" by a highly dubious x-ray image that's been presented now in connection with at least two separate children who don't seem to have been all-but-killed as they should be.
Mid-April did see a brutal new offensive, with a few dozen more killed in rocket attacks on Misrata, as usual including women, children, and the elderly.  Cluster bombs have reportedly been found. The harbor was attacked again. The rebels predicted a total slaughter would finally befall them without more NATO involvement soon.
What they really mean is Misrata will no longer be a rebel town, but the coding helps. The leaders of the US, UK, and France, who happened to be meeting in these same bloody days, jointly denounced, among other things, the “medieval siege” of Misrata. And these pleas finally allowed them to make some new decisions on a core realization that all three nations have agreed on for four decades now - essentially, Gaddafi must go. "It is impossible to imagine a future for Libya with Gaddafi in power," they lied jointly on the 15th. 
Nonetheless, NATO air strikes fell off notably and stayed low-key, for fear of civilian deaths, as government forces pulled themselves fully inside the city. The same situation the rebels had exploited for weeks, when it's protecting the Libyan army, is unaplogetically called "using human shields." This has created the impression that they are snatching civilians and strapping them to their tanks, tying NATO's hands while they drive over piles of young children.
So a decision was made to bring in the drones, well-known for differentiating between civilians and combatants (??) and then the whole situation shifted. (see Misrata: the Tribal Threat)
 Wikipedia. Misrata. http://en.wikipedia.org/wiki/Misrata
 Human Rights Watch. "Libya: Government Attacks in Misrata Kill Civilians." April 10 2011. http://www.hrw.org/en/news/2011/04/10/libya-government-attacks-misrata-kill-civilians
 Kuperman, Alan J. "False pretense for war in Libya?" Boston Globe. April 14 2011. http://articles.boston.com/2011-04-14/bostonglobe/29418371_1_rebel-stronghold-civilians-rebel-positions
 "Rocket barrage hits Misrata, NATO says Gaddafi must go" Times of India, April 15. http://timesofindia.indiatimes.com/world/middle-east/Obama-Sarko-Cameron-seal-deal-Oust-Gaddafi/articleshow/7996431.cms