Grenade power may be the same as ever, but all of the following have changed:
- The damage done at the fringes of the grenade radius has increased.
- It is harder to outrun a grenade, because movement speed has decreased.
- It is harder to jump away from a grenade, because jump height has decreased.
- Without Armor Lock or an extremely well-timed Sprint, you aren't getting away from a grenade any other way.
- Players are more likely to have sub-optimal health in Halo: Reach than in Halo 3, even if they are diligent about hunting down health packs.
Meanwhile, grenades are seemingly as plentiful as they ever were. And while surviving one grenade is a fine thing, an enemy is likely going to toss out everything he has if he's close to death, and if he's not alone, you will often find three or more frags bouncing around you. Given that many people are struggling to aim with the single-shot weapons (which I firmly believe is their failing, not the game's), it's easier to just unload every grenade you have into a space you believe an enemy to be in, especially if you suspect he is weak. Often I just lob a grenade at any red Xs I'm near, because I'm likely to catch someone with their shield(/pants) down.
To boot, the two maps we've experienced so far have a lot of tight, confined spaces that encourage grenade spam.
So, sure, they may be just as 'powerful' in terms of raw damage as they ever were. But the context for that power has changed.