It's just been really fun. I don't mind ObjC but Swift feels so modern and nicer to type. One problem might be that Swift is constantly changing and that means you need to update your code base whenever you want to use the latest Xcode. I don't mind though because that's also a great way to learn about the changes. Ask away if you have any specific questions.
You are not the first person to ask for this so I'm definitely considering it. It's a 1.0 release after all so if you have suggestions then please let me know :)
Sorry that it requires OS X 10.10. I do know that many people haven't upgraded for various reasons. The thing is I wrote the app with Swift and I won't be able to support Mavericks until I can submit the app with Swift 2.0 which lets me target a different OS version. If you want then you can subscribe at the bottom of the page and I'll remind you about Gestimer once OS X 10.11 is out.