>>10138805to list a few:
1) No one-brain-brick gui, only a bunch of LEDs, even the original RCX had a way to select programs.
2) Teal can be hard to integrate with existing collections to make something that looks cohesive.
3) Has 6 ports instead of 7 like the previous version, and while the RCX only had 3 ports, you could stack motors on a single port.
4) Worse specs, it runs at 100mhz, with room for 30 programs and 32mb of storage, while the previous version was 300mhz (and could be overclocked to 400mhz) and had an SD card slot.
5) cables are permanently connected to the motors and sensors, meaning if any part of that cable gets damaged you have to replace the entire thing.
6) No backwards compatability, Ev3 could use NXT motors (but not sensors), and there were first and third party adapters adapters for RCX motors, and the new one is incapable of using any of these.
7) A somewhat more minor issues, but it comes with a 4th motor instead of a touch sensor, severely limiting capabilities out of the box.
All in all, it's pretty crap.
