For example, they both used the same video generator (TMS9918/28). They both used the same audio generator (SN76489). They both used a Z80 running at 3.58MHz. And they both had 1K of system RAM.
One difference I am aware of is that the SG-1000 only had 2K (or was it 4K?) of video RAM whereas the ColecoVision had the maximum the TMS9918 allowed (16K).
What other differences were there? And, other than pin configuration, what prevents me from running SG-1000 games on a ColecoVision?