1. Trello was smart in only taking $10M in VC funding [1]. This allows for a 40x return for it's investors. If Trello were like many other startups, they probably would have taken too much VC money and got themselves in a situation where the VC wouldn't sell unless it was $2B+.
2. Atlassian now has a product that is loved by many developers and business people alike. It will easily be interegrated into their existing stack and it complements their products very well.
TLDR: Both Trello & Atlassian were smart in this acquisition. Couldn't be happier for them (and as a user).
Edit: typos