Up top, just to the right of the Navigation Bar info, is an unmarked slider that adjusts your avie's Z-offset, so you can change how high off the "ground" you are. Great for really small shapes like my pixy alt or for furries with prim feet. The next slider is for your Draw Distance. Very convenient for photos and to reduce lag. I also like the lag meter on the far right. The Inventory slider has a very convenient filter and folder management buttons. The Official Viewer needs these. In the Preferences > Dolphin Viewer 2 > User Interface pane you'll see some really cool options, like Paste as Link (so you can control what is seen in Outfits), and the ability to change tag color options. Not shown because it is inactive is the Voice Speak button next to the Chat box on the bottom. If Voice isn't active, the button disappears to save space.
In the Preferences > Dolphin Viewer 2 > Navigation pane, there are some nice options to control how the Mini-Map works. For example, you can have it show a "Range Ring" that indicates chat distance, 20m in any direction. Having the ability to pinpoint physical prims, like the swimming eel in the Lesbos Mermaid Lagoon, is helpful. These are rendered in green if you own them and in red if someone else owns them. Physical prims can impede avie and vehicle motion so if you drive anything, this is handy. And the Camera option you see is nice for when you teleport into cramped spaces and the view jumps around. I'd still like to see a radar-enhanced Mini-Map like the Imprudence viewer uses or at least an inclusion of a Mini-Map in the People > Near By slider pane, as seen in the Official viewer. I'm sure updates will have the latter.
The best feature of the Preferences > Dolphin Viewer 2 > Graphics pane is the Sculpty Detail slider. Normally one has to activate the Advanced menu then open the Debug tools then type in RendervolumeLODFactor to adjust this number. The higher it is, the faster sculpties fully render. Most creators recommend a setting of 4, but this option only goes up to 3. If you want a higher number then you need the menu settings method. Checking the first box under Alpha Mask Rendering seems to reduce conflicts between overlapping alpha texture prims, much like you'd see in a tree, but I don't think that is what it is for, really. I locked up my whole computer when trying to activate 'Lighting and Shadows' to really test it. Second Life really hates ATI Radeon chipsets.
Preferences > Dolphin Viewer 2 > Miscellaneous isn't terribly special but I like the the two "Other Options," particularly about rezzing objects under the Land's group setting. Countless times just yesterday I had to go back and find objects with the wrong group lest Auto Return became active and everything was returned. The OOC option shown is nice for role play gamers. Just start your chat with (( and type your sentence then hit the Enter key and the viewer will tack on the closing )) to show you are speaking Out Of Character. If your character's name is not the same as your own then you'll still need an OOC device (like you can get from the freebie board at Mystara), but this is a nice option.
The Dolphin viewer also has a really interesting Media Filter option. Essentially it is a firewall for media URLs. When the music stream changes, for example, a popup shows you the new URL and gives you the option to allow, deny, or add it to lists for always play (Whitelist) or never play (Blacklist) before the stream changes. Great for folks who try to slip in spyware URLs. Of course, the lists can be edited. Apparently, the Phoenix/Firestorm viewers are going to get this feature, too.
Also, Web-based profiles are gone. In the Official viewer they are slow and take up a huge amount of screen real estate despite using fonts and images that are too small to be useful. Profiles are back to the the pre-v2.5 viewer behavior, showing up in the side bar. I prefer the separate panes that the pre-Alpha release Firestorm viewer uses, but this rollback by Dolphin is still a big improvement.
Dolphin appears to be fast, solid, and includes nice enhancements over what is available in the Official viewer. The only issue I have, aside from the crash I created while testing as mentioned above, is the wearing of clothes and attachments. Normally if you choose Wear for an item, the previous item worn is removed. With Dolphin, Wear behaves the same as Add ... each item remains on your avatar. The Add function still works, but there is something wrong with Wear.
Other than that, I like Dolphin and I hope to see it improve so there is a variety in the viewer ecosystem, thus spurring competition and growth. If you like to fiddle around with viewers, you should give it a try.