Sets can't have bad cards anymore at common? There have been variants of healing salves and soul feasts since the dawn of magic. You can't just put good cards at every rarity without a number of stinkers. Otherwise the draft environment would quickly turn sterile. Likewise, you can't just print bad cards. There needs to be a proper balance, one that doesn't upset the balance between the five colours. And contrary to popular belief, Onyx Goblet isn't completely unplayable.
Esper doesn't have a mechanic because its synergy is artifacts. I agree it wasn't very developed in the first set, which they amended in Conflux with a variety of 'artifacts matter' cards. There shouldn't be a hard rule that states every faction should have a mechanic. If you impose yourself that, you get duds like Radiance, Replicate and Forecast. It's not like a draft deck only consists of cards of one faction. You can have Exalted creatures in your Naya deck and you can have Unearth creatures in your Jund deck. Cards should synergize better with each other within their mechanic, but there are examples where that doesn't happen (Unleash, Detain, above-mentioned mechanics). It's not the end of the world because in the end it's all about the colours, not the mechanics. Look at Innistrad. Every colour combination has its own identity which isn't tied to a mechanic.
As for manafixing, Shards of Alara had better fixing than Return to Ravnica. The former is also more balanced because all five factions are available from the start, whereas Return to Ravnica starts with 5 of the 10 factions.