Feels weak. I'd suggest going straight up for "Is indestructible" which seems a lot more exciting; unless you're going to have power/toughness switchers too.

P/T switching is one thing to consider, another is anything that cares about toughness. Also I'd personally would like to be able to save a creature from damage but it still be killable.

I like ridiculously high toughness boosts. But Vitenka isn't wrong about 'indestructible' generally being better. Withstand Death costs , which means that this card is either too expensive, or you're intentionally making a 'bad' card.

Hmm Could it still be common if I make it able to target say up to three creatures?

Well, on the flip side - Piety exists.

I'd say "Go ahead, make this "Creatures you control get +0/+5 UEOT" that feels closer to modern power. (Aven Warcraft for example, Bar the Door) - although, well, Akroma's Blessing is kinda the same but a heckofalot better usually.

...Steadfastness. The power level of this effect has been all over the map, hasn't it?

Given that the power level of the effect has been all over the place I'm going to make it up to three target creatures. It should play well with things that care about being targeted.

Yup, this effect is crazily variable. Tower Defense was the one that sprung to mind for me.

This is pretty cool now. I'd definitely play it in my casual Heroic deck.

kinda tired of seeing all these indestructibles and protections. let's go back to basics once in a while. in before Doran.

Psst, Vitenka. Steadfastness is a Sorcery. It's used for a completely other purpose.