It's really hard to say which patents are holding things up. Each technology has many patents. For example, the Selective Laser Sintering Patent is supposed to be a huge one. It has just recently expired (I think at the end of last month). However, there are still plenty of other patents surrounding SLS that make it hard for other companies to enter the market. I do know that when some of the major patents on FDM technology expired, we say an influx of new FDM printers. However, I am not so sure that will be the case with the SLS tech. Same goes for SLA. I don't know which patents on Stereolithography are still active. I know the technology is very old, so I'm assuming most of the major ones have already expired.