From the responses so far it sounds like the choice of theme and game-play rules.
I was actually thinking of evolution in terms quality like lights, sounds, depth and gameplay. The way I see it is:
1. Stern takes over from a failing Sega and start off producing games like HD, SE and Sharkeys
2. They moved up a gear as LOTR, RBION and TSPP were landmark games in that it was only then that Stern showed that they could make good successful complicated games of a good standard. They also investing in top designers for one off games and put more money into the product.
3.That was followed by a mixed bag of games for a couple of years
4. Then the SAM board system marked better sounds better graphics and a bit of physical refinement, with games like LOTR and Spidey, Iron man, Batman.
5. Financial problems saw a some games go to the market too early (Batman/CSI/NBA/24 updates were too late)
6. Then a change to simpler games
7. Then hopefully back to more balanced games with Tron.
Over the years there has been a lot of improvement. Striker Extreme>Tron but I guess unless the theme is right it is all wasted.