Home screen interface and widgets
Stock Android 2.1 on the Nexus has a home screen thumbnail viewer that can be accessed by long pressing on the navigation dots in the corners. You can then select a page to go to it. Sense UI basically took that ability and came up with a really intuitive riff on it. Instead of long pressing on something out of the way, you just do a two-fingered pinch gesture to bring up all your seven home screens as thumbnails. This is sometimes called the " Helicopter View".
Speaking of widgets, HTC Sense has some very nice ones. One of the advantages of heavily modifying Android is that HTC was able to build their own widget framework on top of Google's. So standard Android widgets will run, but HTC's enhanced widgets only work on Sense UI phones. The thing that makes the Sense UI widgets so impressive is that they are animated live. So you get contact and Twitter list widgets that you can flick-scroll through. There are also weather widgets that animate right on the home screen and battery indicators that "bubble" when you charge them. Stock Android phones do not have animated widgets. The widgets on phones like the Nexus One just refresh quickly when you interact with them. For instance, a Twitter widget might have a button to advance tweets in blocks without scrolling.
HTC Sense also adds a completely new function to Android; Scenes. Scenes are a way to save a layout of shortcuts, widgets, and background for later use. You can create a "Work" Scene without all the Twitter and Facebook distractions, and maybe add your exchange account widget. Then use a "Home" scene with all the work-related stuff gone, and the things pertinent to your social life in their place. This is certainly a nice feature for those that are always switching widgets around.
Speedsnappier. In fact, the home screen interface feels much more responsive than stock Android 2.1 because of some optimizations in the Sense UI, as well as some visual tricks like smoothing the screen scrolling. Boot ups tend to be fairly quick despite the extra goodies that have to be loaded.
For the most part, there isn't a huge increase in the speed of apps. Even modified stock apps, like the browser, perform similarly to stock Android. HTC Sense won't magically make the CPU faster. In fact, with seven tempting home screens to cram full of widgets, the operating system could be slowed down quickly. Overall speed increases are one of the chief benefits of the Cyanogen Mods. Many users have reported that their stock Android phones run considerable better with these custom ROMs.
The keyboard and keypad
One frustration with the stock Android keyboard is that you have to open the alternate keyboard to add most special characters and numbers. The Sense keyboard, however, assigns each key an alternate character that can be accessed via a long press. This is a welcome addition for those times we just need to add a number without opening the alternate keyboard. The dialer keypad is improved as well. In the phone app, you can just begin typing out a contacts name T9 style, or their number. The list above the keypad will be automatically filtered as you go.
Flash in the browser The browser on Sense UI phones is very similar to that of stock Android phones. The big difference here is the support for Adobe Flash Lite that's built in. Most newer HTC Sense devices have Flash Lite version 4.0. Flash Lite won't play all Flash content you come across online; it's actually fairly limited in what it can do. A Flash based audio streaming elements might work fine, but video tends to get choppy in most cases.
Adobe and Google are working to integrate Flash 10.1 for mobile into the upcoming Android 2.2 Froyo update, and it's unclear what that means for HTC's browser. Stock Android phones like the Nexus One and Droid will have the full Flash implementation, but it's unclear if HTC Sense phones would be able to take advantage of it.
While some phones like the G1 (HTC Dream) have been left in the dust, we're expecting the newer stock phones to be updated for some time to come. So the Incredible might be running the newest version of Android right now, but when 2.2 is pushed out to the Nexus One and (eventually) the Droid, Incredible owners might start wondering where their update is.
These are the things that make a Sense UI phone special. It goes quite a long way to clean up the Android experience with UI enhancements and new features. There are some concerns mostly relating to dependence on HTC for systems updates and features, but for most, it is worth the trade off. Overall, Sense UI is a more attractive Android implementation that adds functionality. We see no real reason to warn you away if you're planning on snapping up an HTC Incredible or EVO 4G if you like the look of Sense UI. Do you prefer stock Android or HTC Sense UI on an Android phone?
Image credit: Flickr user misteryaya, HTC