Every set will introduce two or three new mechanics. Some of the cards from that set will be entirely based around those mechanics (for example, Vehicles as artifacts that become creatures when "crewed" by other creatures). Many cards will be staples needed in every set with a new mechanic attached (e.g., "Target creature gets +2/+2 and gains X until end of turn"). Also, sometimes mechanics are tweaks and improvements to old ones that didn't pan out well.

I listen to the lead designer's (Mark Rosewater) Drive to Work podcast. He's worked for MTG as a contractor or employee since the mid 90s. He's mentioned this problem a couple times. His solutions: (1) See MTG as a basic rules system for playing different kinds of games. Makes sense, considering it has multiple win conditions and methods of achieving them. (2) With each set, focus on a theme and dive as deep as you can. There's been a "World's Fair steampunk" set, a titanic Lovecraftian monsters set, a Greek mythology set.