Here's a list of the key differences (taken from my own blog at http://www.recantha.co.uk/blog/?p=10323)
- 4 USB ports – for the first time, you’ll be able to have a keyboard, mouse and wifi dongle plugged in without needing a powered hub. Bear in mind, you will probably need a more powerful power supply for the B+ if you want to use all 4 ports. They’re recommending a 2A supply. Of course, if you’re just using a mouse and a keyboard then a 1A supply will probably be absolutely fine.
- Better USB hot plug capability – you should be able to plug in your wifi dongles in without reboots!
- A new Ethernet port with activity lights.
- The USB and Ethernet ports are now all in a straight line, rather than in the B’s staggered arrangement.
- The yellow composite video port has gone and there is now a combined 3.5mm jack that provides audio and the analogue video signal.
- The HDMI port, the CSI (camera) and DSI (display) ports have all moved slightly.
- The micro USB power socket has changed sides. This means that all the ports are on the same side of the Pi – so cable management is much better with the B+.
- The B+ sports a 40-pin GPIO header (compared to the B’s 26) and no longer has a P5 and P3 header. All the GPIO is on that one header now. The 26 pins to the left (nearest the corner) are backwards compatible so all your add-on boards will work (with the exception of any audio boards like the Wolfson which requires the P5 header which is, of course, not on the B+).
- 4 mounting holes, conveniently located on the corners of the board rather than the 2 oddly-placed holes on the B.
- On the back, the B+ uses a micro SD card (as opposed to the B’s full size SD card) with a push-push action (i.e. you push it in and it locks in place, you push it again and it clicks out). The card still protrudes slightly (presumably to make it easy to remove) but the (metal) slot is a vast improvement on the flimsy plastic of the B.
- Much clearer silk screen labelling in general.
- Lower power requirements – it will use between 0.5 watts and 1 watt less than the model B.
- Much better audio output thanks to a dedicated power supply.
- Rounded corners! (Still doesn’t fit in an Altoids tin but who cares! The aesthetic change is welcome)
In terms of doing new things with it, obviously you've got more GPIO pins, so if you want more inputs or outputs then the B+ is for you. Plus it's much easier to plug USB things in to (less need for a powered hub). If you're doing robotics with a portable power supply, the B+ will last longer on it thanks to the lower power consumption.