Thatcher did some good things whilst PM. These can be listed as follows:

1) Curbing the power of the trade unions, which were corrupt, out of control and bad for the British economy.

2) Winning the Falklands War.

3) Pursuing the path of negotiation in ending the Cold War and providing valuable international support to Reagan.

That's about it. Balanced against that you have:

1) Raising unemployment to the highest level in Britain's post-war history and never really fixing the problem. We have never had unemployment as low as it was before Thatcher came to power.

2) Deregulating the banks and finance sector, letting them give out credit like it was free candy. Whilst Thatcherism wasn't behind the global economic crisis, the legacy of her policies ensured that Britain was hit harder by the consequences than almost any other Western country.

3) Giving people the opportunity to buy their own council homes, which was a short-term good move which led to a long-term crisis in social housing supply (which is now in danger of becoming a fully-blown catastrophe). The lack of social housing in the UK has led to a mixture of people living in absolute squalor and the taxpayer having to pay private landlords obscene amounts of money.

4) Shedding Britain of jobs for unskilled workers and instead promoting jobs in the financial sector. This has resulted in jobs for the unqualified, non-university-attending poorer part of society evaporating in favour of jobs only attainable by those rich enough to go to university (although this problem was actually made a lot worse by Blair, who brought in tuition fees).

5) Giving unqualified support to the mass-murdering dictator Augusto Pinochet even after the sheer scale of his crimes became apparent.

6) Being soft on apartheid. She suggested this was to stay engaged (and thus influential) with the South African government, but by not condemning apartheid she gave the impression that Britain supported it, which was disastrous for the UK's reputation in the area (already not good because of the whole colonial situation).

7) Being resolutely and publicly homophobic.

Being tainted by association with the Hillsborough disaster and subsequent cover-up.

I'm certainly not dancing in the streets to celebrate the death of a sick old lady, but suggesting that Thatcher was good for Britain is to only take the very short-term view, in which Thatcher did things that immeasurably improved the standing of Britain in the world but at the cost of long-term financial stability. In essence, she took power from the corrupt and too-powerful unions and gave it instead to corrupt and too-powerful banks, and neither Major, Blair or Brown did anything to curb their power, directly resulting in how hard the global economic downturn has hit the UK.