Let's set aside for the moment that we have quotas at all, and look at excess charges. Instead of just slowing you down, a number of ISPs will charge you excess if you go over your monthly quota.
For many of them, the pricing is quite reasonable - say a few dollars per gigabyte. But for some, excess charges are a massive trap, with the ISP hitting you with over-the-top pricing for the misdemeanour of exceeding your quota.
It's especially prevalent on the cheapest plans, where you're most likely to exceed your monthly quota (and anybody who thinks that 200MB per month is enough is kidding themselves - a single Windows update might blow away that and more without you even knowing it).
Telstra is the poster child for this kind of practice, charging 15c per MB for excess usage on cable and ADSL services. That's $150 per GB. It's not alone, though - stupid excess charges are endemic to 3G broadband in particular.
Also in this series, How to Get a Good Broadband Deal
Part 4: Bundling deals
Part 3: beware pre-paid wireless metering
Part 2: Get a "reserve tank"
Part 1: Naked, bundled, pre-paid
You can read more about wireless plans in our Broadband Buyer's Guide: Fixed vs Mobile