Glamour Bard - Mantle of Majesty Question

Hoping to get the collective wisdom of the group on the Mantle of Majesty ability from the glamour bard ability's. 

OK, so rules as written:

Mantle of Majesty

As a bonus action, you cast command, without expending a spell slot, and you take on an appearance of unearthly beauty for 1 minute or until your concentration ends (as if you were concentrating on a spell). During this time, you can cast command as a bonus action on each of your turns, without expending a spell slot.

Casting time - Bonus Action

A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven’t already taken a bonus action this turn. You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.

So, if mantle of majesty is in effect and the Bard casts command as a Bonus action does that mean the bard cant cast another non-cantrip spells as an action, PLUS while MoM is in effect if they choose not to use Bonus Action command they cant cast a different concentration spell without dropping MoM? Or would the casting time rule not apply here? (Baldur's Gate 3 may have ruined me for this ability)

Depends if you're playing the 2014 or 2024 one- in 2024, rather than checking timing, the rule is you can only cast one spell with a slot per turn. Thus you can still cast other leveled spells on the turn you use this feature. For the 2014 version, your interpretation is correct- the fact that you cast Command as a Bonus Action would preclude you from casting any other leveled spells on the same turn.

Appreciate the clarification, all.  Glad this prompted the interesting side conversation.

Quote from SunIsGettingRealLow96589 >>

 "you can only spend one spell slot per turn on casting a spell."

I can think of "FeyTouched" as a feature that lets you cast a spell (MistyStep) without using a slot once per long rest. 
but I'm tired and its late and I can't think of any features right now that arent' a spell but still burn a spell slot?
moving sorcerer metamagic from slots to points, I guess, would be one. I don't think I would have counted that in the one-slot per turn limit.

The Paladin for example have class features that uses this mechanic.

Holy Nimbus

As a Bonus Action, you can imbue your Aura of Protection with holy power, granting the benefits below for 10 minutes or until you end them (no action required). Once you use this feature, you can’t use it again until you finish a Long Rest. You can also restore your use of it by expending a level 5 spell slot (no action required).

Quote from SunIsGettingRealLow96589 >>

 " If your Concentration is broken, the spell dissipates without taking effect."

hm.... I read that as the fireball doesn't produce fire, but also doesn't use a spell slot. But burn a slot at Ready, and maybe nothing happens at Reaction works too.

Weirdly, I think WarCaster could burn a slot on an opportunity-like-spell-attack...

The spell slot is expended when the spell is cast. For a Reaction spell that happens when you take the Reaction to cast. But for a readied spell that happens when you take the Action to cast it (i.e on your turn).

Quote from Davenader >>

 

So, if mantle of majesty is in effect and the Bard casts command as a Bonus action does that mean the bard cant cast another non-cantrip spells as an action, PLUS while MoM is in effect if they choose not to use Bonus Action command they cant cast a different concentration spell without dropping MoM? Or would the casting time rule not apply here? (Baldur's Gate 3 may have ruined me for this ability)

 While you can still expend one spell slot to cast a spell after casting Command without expending a spell slot, if that spell has a duration that requires Concentration, you will lose Concentration on your Mantle of Majesty effect.

Quote from SunIsGettingRealLow96589>>

 " If your Concentration is broken, the spell dissipates without taking effect."

hm.... I read that as the fireball doesn't produce fire, but also doesn't use a spell slot. But burn a slot at Ready, and maybe nothing happens at Reaction works too. [...]

 It was already explained in the thread, but you also have this in the SAC:

How does readying a spell work? Do you lose your spell slot if the trigger never occurs?

When you take the Ready action to ready a spell, you immediately expend any resources used to cast said spell, including any spell slot. You don’t get any of these resources back if the trigger never occurs. See appendix C of the Player’s Handbook.

"you can only spend one spell slot per turn on casting a spell."

I can think of "FeyTouched" as a feature that lets you cast a spell (MistyStep) without using a slot once per long rest. 
but I'm tired and its late and I can't think of any features right now that arent' a spell but still burn a spell slot?
moving sorcerer metamagic from slots to points, I guess, would be one. I don't think I would have counted that in the one-slot per turn limit.

 Some examples (EDIT: it isn't an exhaustive list; see wagnarokkr's answer below for another one; EDIT2: and Plaguescarred's):

- Level 5: Font of Inspiration
- Level 5: Wild Resurgence
- Level 3: Primal Companion (Restoring or Replacing the Beast)

Other feature effects that let you expand a spell slot without actually casting a spell include;

• Level 06: Mantle of Majesty (Bard)
• Level 14: Moonlight Step (Druid)
• Level 20: Living Legend (Paladin)
• Level 20: Elder Chamption (Paladin)
• Level 20: Avenging Angel (Paladin)