Direct access to timing and fuel maps, narrowband simulation of wideband (this means you can plug the wideband in and borrow the signal from it and unplug the stock 02 sensor, even on a 1G - I don't know if you can specify the output range used so you can run lean, or if it can interface directly with a wideband probe or not though.)
Also, full RPM based maps up to 10,000 RPM (instead of just extrapolating from the 7500 RPM slider), and increased load tables (up to about 30psi boost)
Other minor things like improved datalogging datarate, integrated support for the Evo 8 MAF, native support for USB, Linux and Mac and upgradable firmware.
Really the only differences between V3 and what most people use a standalone for are that it is platform specific to DSMs. Obviously there are still things standalones can do that it can't (speed density, complete customization, etc), but for the price?
Keep in mind that the offer of firmware upgrades means that there will be freedom to introduce completely new features (like speed density, etc. if they get around to it)
I'd say unless you already have DSMLink V2 dialed in, and don't have any problems or things you wish you had, and don't plan on making any more upgrades, it's definitely worth the $175 for the upgrade