I love the concept of the random city level up bonuses. I don't think there are uncommon and rare traits though. I don't think I have ever seen a level 1 or 2 rare trait. You end up with mostly the same choices every game which does make the feature kind of boring. Not to mention the bonuses themselves don't do enough to differentiate or specialize a city.

Sample traits:

• Level 1 - Rare - Allows you to build a Cantina which gives you +1 to prestige and +1 gold a turn. A little powerful for a level 1 building but hey it is rare and will give you a nice boost if it ever comes up.
• Level 2 - Rare - Allows you to build a Safehouse. Champions in this city gain have a 50% chance to gain 1 XP per turn. Stacks with the Adventurer's Guild.
• Level 2 - Uncommon - A random neutral camp spawns near your city. (Mercenaries, or ogres, or maybe nagas.)
• Level 3 - Allows you to build wonder that increases research by this city 30%.

Basically there needs to be something to get excitied about.