I like the idea of incorporating estates, holdings and followers/troops as paragon and/or epic tier rewards. Yes, many DMs do this already, but one valid criticism of D&D (particularly 3e and 4e) is that there are chapters of rules on combat and stabbing things in the face, but there is little in the mechanics to encourage rp and story. Sure many groups do this or house rule, but I like to think of the new teen having only played mmorpgs sitting down with this new D&D next book he just bought.
Keeps would be a great way of saying to new DMs, your paragon tier Fighter doesn't just move to Northrend to grind the new higher lvl mobs, you can mix up the game and let them become rulers and champions.
The original rules on Keeps were fairly vague - roll on a few random tables to work out how good your troops would be etc. The only detailed D&D rules on running a Kingdom I recall were in Birthright (which I thought was cool, even if it tanked).