Size is the biggest difference between poker sites regulated at a State level and the international ones. At the time of writing the biggest real money poker site in the US (the combined WSOP.com NJ and NV rooms) see peaks of cash game traffic at 300 players. Compare this with the biggest international site (PokerStars) which sees peaks of more than 14,000. You can view the traffic stats here.
You’ll find the best US poker sites have partnerships between brick and mortar casinos and big international poker sites. This means that the software is similar to that of the parent sites – though the number of games and guaranteed prize pools for the tournaments is much lower.
As more states legislate, and player pools are joined together, it is possible that a regulated USA poker site will match the bigger international ones in size.