Diversity of features. It covers calendar integration, task management, expense and time tracking, client estimates and invoicing is integrated, discussions with clients are threaded and neatly organized at all times, file uploads that act as a mini-Dropbox for yourself and your clients, notes, facebook-style timeline of the activity within each project... And I haven't even touched on the level of customization, multitude of extensions (Zapier included), scalability (with self-hosted options!), and the iOS app that works with either cloud or self-hosted versions (our clients LOVE the app).
We love ActiveCollab because of it's diversity. That's the one reason that in our opinion, it's simply irreplaceable. Having to replace it would be like adding tripling the administrative work on our plates. It replaces the need for multiple separate services that do each of those things separately. Who wants that when you can have one single login to one single system that can rule them all? ;)
The iOS and Android apps needs some work. They don't do as much as the desktop version, so they feel limited. They don't really take advantage of features like gestures or multi-touch, which every smartphone has.
We'd really like to be able to upload more than just pictures from the camera roll; some kind of file picking, or option to upload to ActiveCollab from another app (share sheets) would go a LONG way. Optimization for tablet-size screens would be sweet too, along with the simple ability to make text in the app bigger or smaller, so it's easier to read.
The calendar allows for syndication via subscription links, but stopping there feels premature. It would help a lot to be able to integrate to Calendar apps (via CalDAV), since the calendar isn't available on the mobile app either. It would be really nice so that we can add or edit events from our native calendar apps of choice.
Support is fantastic, and extremely knowledgeable. We run a self-hosted installation, which is more economical for us, but we don't have pros running our server environment. That task falls on us, teaching ourselves as we go, and it feels like support will drop clues about what an issue is, instead of going the distance and going in themselves to help fix the problem. Granted, that's more of a problem with /us/, since we're business administrators and not experts in server management. But support will not abandon you. They've always got your back, no matter how tough the problem!