1) This is mostly AT&T putting a stake in the ground now to be able to charge more in the future. Yes, 98% of subscribers currently use less than 2GB of data but that won't be true in 2-3 years. Instead of increasing those 200MB/2GB limits carriers will offer additional plans and reshape/abolish existing ones to nudge people into upper tiers. As happened with minutes and SMS, you'll pay more for less as time goes on, even as the cost to carriers plummets.
2) I don't know what sort of bandwidth VOIP uses, but I bet it's a lot less attractive as an alternative to regular calling with a 2GB limit.
3) Tethering is also a lot less attractive there's a 2GB data limit. Note that extra $20 doesn't buy you any more data, it just allows you to use your existing data in a different way. That really really stinks.
4) The overage charges also stink. On the $25 plan you pay $10 for an extra GB, on the $15 you pay $15 for an extra 200MB. That's just nasty.