I think over the years, the parts rules have become significantly less restrictive, while the game rules have become significantly more restrictive.
Parts rules:
Most teams have more power in their drivetrains now then entire robots used to.
Prior to 1998, robots ran on two rechargeable drill batteries, and FIRST charged them for you. You swapped them after every match. 1998 introduced the SLA motorcycle batteries.
You used to be only allowed to use parts that were specifically allowed (for example, a single 4x8 sheet of plywood), or that could be purchased from
Small Parts, with strict price limits (<$425 total). Since then, they've significantly increased the price limits and opened things up to any supplier. There also used to be a list of specifically excluded "exotic" materials.
Game Rules:
Intentional tipping and intentionally detaching mechanisms were made illegal after 1997.
Bumpers were allowed in 2000, the concept of standard bumpers were introduced in 2006, and were made mandatory in 2008.
Since 2006, there has been some limit on the size of the robot not only before the match, but during the match. Compare that to some of these robots:
http://www.chiefdelphi.com/media/photos/14988 and
http://www.chiefdelphi.com/media/photos/13784
Wedges close to the ground have been illegal after 2005.