Moment of Glory provides all characters with resistance. This power grants ONE character resistance as a daily power.
That's still enough to render that character incredibly tough (math-breakingly). At-will attacks are doing less than half damage typically (typically 18 at 10th-level, you're only taking 8), and it's worse if there's ongoing damage involved (since this is usually affecting two low damage rolls twice; flat ongoing 30-type attacks are incredibly rare, probably due to the saving throw system making that too swingy).
Paragon auras typically deal 10 damage, which this completely blocks. A 10th-level minion is supposed to do 9 damage according to the guidelines, but they typically do a little more; still, blocking more than 75% of the damage that a minion does is more than a little problematic.
Monsters that have retaliatory auto-damage (think the tiefling racial power or the paladin's varying punishment abilities) will see their special powers laughed off.
Certain monsters use multiple weaker attacks rather than a single strong attack (the tembo is a good example, it can get four relatively weak attacks in a round not counting its aura).
This power by itself would seriously limit what you can use in an encounter. No ongoing damage, no traps (since those are usually at-will only), no dangerous terrain that deals damage (drowning in silt is fine though, if you're aiming for the restraining condition and not drowning "damage"...), no tembos, no monsters that have a minor action and standard action that team up to equal standard damage, no paladin-type opponents...
Or alternatively, it limits what sort of encounters DMs can use until that power is used. I don't want to save all my paladin-type monsters with auras and ongoing damage attacks until the final encounter of the day (if the day ends up that way), and it'll be worse if the PC saves that power until the end of the day.
For the sake of comparison, at level 11, thri-keen can gain resist 2 all with a feat while bloodied (from Dark Sun).
A paragon level knight can, with Armor of Conviction, get resistance while bloodied too. That's not on all the time (since you have to be bloodied), and it's only resist 5 all. By 16th-level, even minions are trivially cutting through that.
Warding elemental, again available at level 11, give resistance 5 all, to all allies within 2 (again from Dark Sun).
That actually seems too good, since an animist shaman can have that active all the time. However, cutting damage by 5 isn't going to completely nerf ongoing damage or minions.
Shifters can gain resist 2 all while regenerating at level 11 (PHB2).
IIRC, shifters can only regenerate once bloodied. I could be wrong, I don't have a shifter in my game (it's Dark Sun, there's no shifters there). I certainly hope that's the case, because uncapped regeneration is far worse than regeneration while bloodied for PC balance.
I really don't think one character gaining resist 10 for one encounter a day will make the game unfun. But your mileage might vary.
I'm comparing it in part to Bloodcut Armor, which gives you equivalent resistance ...
for one round. Grindiness goes both ways. It's quite possible for a DM to design an unkillable/grindy monster (or use one, I hear bad things about insubstantial weakening wraiths) and it's generally frowned upon/avoided. That doesn't mean it's impossible for a player to do the same thing for a PC.
You could also compare this to Stoneskin, which I'm noticing is basically the same thing but 6 levels higher. I don't know if I like that actually; at least that power should be taking up a sustain minor (sort of like how Fly and other such powerful effects allow for that). My campaign hasn't reached enough levels to worry about Stoneskin. (Also no wizard PCs, which is a bit disappointing actually.)