There is nothing really key about 100 days. It is a purely arbitrary point at which to assess a government.
However, one key assessment is what the interest rate is on government debt. The figures from Bloomberg today are:
UK (10 year) 3.037
Ireland (10 year) 5.236
Greece (10 year) 10.692
Germany (10 year) 2.353
That affects in the long term how much money is available for public services. Labour's strategy of don't cut so much in the short term leads to larger cuts in the Long term.
In the mean time the Coalition government is working to make the UK a nicer place to live. The banning of demanding money with menaces (aka car clamping on private land) is a good example of that. Yes people may still have to pay penalty charges, but the behaviour of so many clamping operations has been totally unacceptable.