I haven’t actually used them yet, but from a search I did some time ago I was considering using a Surface Pro as a data entry device, I found the previously mentioned Zoot, MyInfo and Mybase to be the closest alternatives, in terms of offering both file management and note taking functions. They have some equivalent features to replicants in that you can save items into different collections/folders without duplicating them - a feature I find essential in DT. Mybase also works on Mac, and apparently the upcomming MyInfo version 7 will offer more cross-platform possibilities.
If you can dispense with DT3 wonderful custom metadata fields, Zotero would be a strong contender. It’s open source, cross-platform, offers extensive tagging features, and in my experience it offers a very solid performance. I like that you can add notes to any item and the notes will show as if the item was a group or folder. By pressing the option/alt key, all the collections that have an item are highlighted. It lets you filter entries by easily selecting multiple tags from the tags pane. It has a “related” pane, where you can link related items, and with Zutilo you can have unique URLs of each item for deep linking with other applications. But the metadata is really oriented to reference management, and the fact that you can’t create custom metadata fields is to me a crucial limitation.
Finally, I’ve been recently very interested in iMapping. It’s a very different approach, not really a DT equivalent, but it has the mind mapping and whiteboarding features that I’ve always missed in DT. Since it’s a Java app, it’s cross-platform. I won’t go into its features now, but it is a really well thought piece of software.
I share the preference for Mac’s design that have been pointed out. In addition, I get the impression that software runs more efficiently - with less resources you do more. But there’s a niche I see Windows having a current advantage, and that’s in the Surface Pro devices - full desktop OS on a multi-touch, portable, convertible device with great handwriting possibilities. iPads, now with pencil integration, are great sketching interfaces, but the software is still not on par with the desktop equivalent. You see this with DTG. It does the job for note entry and basic browsing and search, but it’s still far behind in many features of DT3, like custom metadata, filters and advanced search. And something similar happens with other apps.
Having said all this, DT being mac-only is one of the main reasons I’m not using Surface Pros, given that so much of my workflow is now related to DT given how robust it is.