Now I'm not going to say that saving thousands of lives in one go is unimportant, because...obviously, it is! However, it's not at all the same thing and the presentation of Tsunade's village-saving act was wholly unsatisfying. The end result felt a lot more like an excuse to get her out of the way.
Of course they were having none of that (oh chuunin why do I ever go to you, all you do is cause me pain) and I wound up feeling extraordinarily frustrated after arguing with them for a while.
Now I realize that there was a comparison I could draw, to similar events in another shonen manga. A better manga! A manga called...Fairy Tail.
Since many of you are probably less than familiar with Fairy Tail, here's the relevant details. Fairy Tail is a mage guild. One full of hot-blooded shonen types. Aside from the guild's Master, Makarov, there are four members who are contenders for the title of Strongest In Fairy Tail: The Master's jerkass grandson Luxus, the reclusive Mistgun, Gildartz (who has never even been shown on screen), and Erza, who basically acts as the guild's second in command because the other three are incredibly anti-social. Erza is Tsunade's counterpart in our comparison.
So, Luxus decides that he's done waiting for grandpa to pop his clogs, and is taking control of Fairy Tail NOW. To do that he forces a conflict between him and his three followers and the rest of the guild by taking hostages. If they can't find and defeat the four of them within three or so hours, the hostages will be terminated. They use magic to trap Master Makarov in the guild hall, and then fight it out...
Anyways a bunch of stuff happens that I'm not going to describe here because I'm lazy and it's not relevant, but here's the situation at the relevant point in canon: The majority of Fairy Tail, and all of Luxus' followers have been eliminated (unconscious, not dead). Only Luxus, Erza, and Natsu and Gazille (two members of the guild who are on the tier right below Erza and Luxus in terms of power) are currently able to fight. Oh, and there are 300 magical crystals floating in the sky above the town. In about two minutes they're going to rain lightning down and kill everybody. If they're destroyed, the damage will also reflect onto the person who destroyed them (which happened twice so I guess there were actually 298 crystals).
To this point, Erza did not know about the Hall of Thunder, which is what the crystals are called. When she finds out, she tells Natsu to take care of it. But he can't, because once he takes one of them out, it'll fry him good and he won't be able to get the next 297.
I think you can see where this is going.
So, Erza leaves Luxus to Natsu, and runs out to destroy the Hall of Thunder herself. Much like how Tsunade saved the villagers and then left Naruto to fight Pain. Except not shitty, and now let's examine why.
The first big difference is that at no point do we actually see Tsunade do anything. Pain's massive Shinra Tensei hits, and then we see people still alive, and some ANBU dick comments that it was because of her...but she never does anything. In fact, she seems as confused as anybody else. In fact, reading the aforementioned ANBU dick's internal monologue, it doesn't seem like she did anything at all! Her slug did all the work, she just provided it was chakra. She wasn't an active participant, she was a battery.
This difference is important! If we don't see the character making a choice, don't see them put their plan into action, if we don't even see them acknowledging that they did it on purpose (and in fact have dialogue suggesting that she didn't), the action seems less like something they did, and more like something that happened to them. One is empowering, if nothing else than in that it affirms her decision-making skills, and the other is disempowering (is that a word?). That difference is critical to how we read the scenes, and how it affects the reader's impressions of the characters.
The second major difference is that Erza's village-saving accomplishment is easier to measure in terms of power. It is no doubt, from an in-universe perspective, an impressive feat to be able to provide enough chakra to the slug to have him protect all the villagers. However, for the readers, that's difficult to quantify, especially since so little time was spent on it. The amounts of chakra have always been kept vague; which is fine by me, the last thing this manga needs is more numbers being thrown around. But in the absence of such information, all we have to go by is the visual punch of the two...and Tsunade's accomplishments leave much to be desired there. In fact, they leave everything to be desired, since we never see her DO any of it!
The third major difference is that while both Tsunade and Erza have brief altercations with the arc's villain, Tsunade's hurts her credibility as a combatant while Erza's does not. Tsunade just stands there blankly while Robopain bears down on her, requiring Naruto to save her ass. Erza, not so much. She doesn't fight Luxus for very long, but as soon as she wades into the fray, Luxus' attention is entirely on her. Natsu is an annoyance to him; Erza is a threat.
It's certainly understandable that Kishimoto may not have wanted Tsunade to be the one to fight and defeat pain. He clearly wanted to give that job to Naruto, and fair enough. But there's ways of doing these things, and Kishimoto chose the wrong one.