In my opinion, you absolutely must give people the chance to try your product. This means having a trial build on your website, which most Mac indies do but it's a bit problematic for paid apps, as I presume a lot of people on the MAS itself are unaware. It's in the best interest of everyone to have trials - for customers, so that they don't feel duped and actually buy a product they're happy with and for developers, so that they acquire real customers, not play a game of "can we convince people to fork over money without trying".
Moreover, it's in the best interest of customers to actually provide paid updates - they can vote with their wallet whether a business should survive. But without the proper availability of paid updates, customers are left with no choice, even if they did want to support developers. Unfortunately, the voice of a vocal minority instils fear in developers to try and charge for such updates.
Given the current climate, our plan for the future is to lower our initial price and offer major versions as paid updates. Instead of, say, charging $40 for the current version + $20 for an update, we'll just always charge $30 for the current version. As a side effect, it lowers the entry for everyone and your real customers can still vote with their wallets and support your efforts, if they choose to do so. One of the downsides is the introduction of a new SKU which is not that much of a problem (before the MAS, you had to do the same anyhow). It's usually wise to provide a way for customers to stay in touch with you and hear about updates but only if they want to (don't smack dialogs repeatedly in their face, that's just not nice).