Transmit's droplets are icons you can effortlessly create that allow a quick DnD operation to send files/folders up to a specific directory in one of your Transmit FTP accounts. What's even nicer is that you can opt to save the relevant password in the droplet, or require it to be input each time you use it to upload something. Creating a droplet is simple: right-click (control-click) on a directory in Transmit and chose the "Save droplet for Folder..." option like I have pictured.
The one drawback of these droplets is that they still have to start Transmit in order to actually perform the upload. On the flip side, however, they also automatically quit the app when finished. Ultimately, these droplets are incredibly handy if you have multiple FTP accounts with various directories into which you simply need to dump stuff.
Another new feature is Growl support. Since I am a lover of Growl, this is most welcome, but Transmit's implementation is a little quirky as there is no option in Transmit to toggle this feature; it's all done through the Growl System Preferences pane. Heads up to those who are new to the realm of Growl.
The last major new feature in 3.5, the one that truly made this love affair flourish, is the "edit any remote file, in any application" feature Dave mentioned. This one almost has to be experienced to believe: you can now double-click remote files in Transmit (such as image or text files) and it will download and open them in an external editor you specify in the preferences. You can then work your magic, simply hit "Save" and Transmit uploads the altered file - seamlessly (try this on an image file with Photoshop for an insta-drool effect). Gone are the days of "download, surf to your local directory, open the file, edit, save, then upload." This alone was worth purchasing a license.
Which reminds me: Transmit offers a 15 day trial, while a single license is a bargain at $29.95. Bulk pricing and upgrades are available.