Having laws tested by courts is not a very special idea; it's exactly what the Supreme Court in the United States does. Other countries have different systems; the Upper House (e.g. House of Lords in the UK, or Senate in many European countries) do this. Neither system is perfect and both have their up- and downsides, but these systems exist for a reason. You really want to make sure the basic principles of your democracy and society are upheld – as codified in the Basic Law in Israel – and can't "just" be overruled with a simple 51% majority just because this one election swung that way.
Some things could probably be reformed; that judges choose their own replacement for example – although making this a political process also has its downsides, as we've seen in the US – but that's not really what people are worried about. The problem with the current proposal any checks and balances can be overrides with little more than a shrug, reducing the court to an advisory organ rather than any real "checks and balances" power. And what's worse all of this is done over a fairly small set of issues – mostly Jewish settlements and some related issues – which smells of scorched earth politics.
And that the whole "Israel is for the Jewish people" Basic Law got approved by the courts proves it's not as left-wing biased as is claimed.