Here's one of the things that struck me as nice about Toradora's ending:

Many ordinary romantic comedy animes have endings that could immediately be followed with 'and then complications ensue' (with 'stay tuned for the second season' being optional). While the leads have gotten together and the show has ended, things have not really concluded.

Toradora's ending is not like this. It stretches events out long enough after the climactic events to make it clear that things really are concluded and that complications did not ensue and, I think, is pretty firm about it. (For me, part of that firmness is the mood of the very end, including the voiceovers.)

(One measure of this is that I think a second season would be either very bad or very different. There is just no story left after the ending; either you get a different one, or you're reviving a zombie.)