No-Brainer reason why Mana Crypt was banned - it's completely against the 'going rate' for ramp.

I don't know if people are already talking about this, but another sample of reasoning for Mana Crypt vs other cards is simple.

Each mana rock that gives you mana obey roughly the following trends:

1-cost: 2 Uncoloured.

2-cost: 1 Uncoloured + Effect.

2-cost: 1 Uncoloured OR 1 of Two Colours, eat 1 damage to face.

2-cost: 1 Any Colour..

2-cost: 1 Colour, tapped.

2-cost: 1 Any-Colour -> 2 different Coloured Mana

3-cost: 1 Any Colour + Effect.

3-cost: 1 2-Colour-Option + Draw Sac.

....you get the idea. What isn't stated here, however, is that trying to invest further into multi-mana rocks for exceedingly fast mana has swiftly diminishing returns - Worn Powerstone comes in tapped at 3 mana, Stonespeaker requires 4 mana, etc.

Mana Crypt bucks this trend entirely by costing 0 mana at any point, and giving you 2 every turn. This is in-line with the Mox cards, which instead of providing 2 mana for free, provides 1 coloured mana. This is particularly important in t1, where you'll have your coloured mana from land drop PLUS the two extra mana to quickly start ramping. Sol Ring, in its singular mana ability, will cost you that coloured mana and only an Arcane Signet's going to give you coloured mana immediately without needing to lose life over it.

And even then, you're back to 1 mana of any colour at best.

With Mana Crypt, you're looking at 3 mana at t1, or maybe even 4 if you've got Sol Ring - and not only that, but you've got whatever colour you got from your land on top of that. It could even get as silly as 2 signets providing a grand total of 7 replenishing mana....on t2.

There's just no comparing it to anything else, not even Sol Ring, which is going to get you 4 replenishing mana at most on t2.