We experimented with Trello at our company before we bought into Jira, and the attempt fell flat on its face because of Trello simplicity. It's hard to describe the difference, but in a nutshell Jira was better at supporting collaboration, capturing knowledge, and supporting subtly different workflows that different teams required.
Based on that, it's kind of a neat move by Atlassian. Now they have an instant base of potential new Jira users. As these Trello customers grow and find out that their needs can no longer be met, now there is a clear upgrade path.
Presumably they will build some migration tools to make the upgrade seamless/convert the database of tickets. Or they could even build in cross-product integration so you can embed links to Jira tickets in Trello cards, or vice versa.