In my experience & opinion, there are two primary reasons this game is widely hated (mostly by people who have never played it): 1. It was so heavily marketed and hyped that no game could have possibly met the expectations, and 2. The pits into which ET often falls in the game legitimately were frustrating, especially the imperfect detection which caused you to fall into the pit even when on screen there was clearly no pixel overlap between player and pit. Aside from the oversensitive pits, the widespread hate for the game is misplaced, misguided, and more of a result of social behavior than anything in the game itself.
The E.T. game combines elements of various previous successful Atari games: basic structural and perspective elements established by Warren Robinett's classic "Adventure", the complex multi-screen predictable larger world pattern of "Superman", the "gather the pieces to complete the Mcguffin" gameplay mechanism of "Haunted House", etc. It resulted in a game which was broadly similar to Howard Scott Warshaw's previous game, "Raiders of the Lost Ark," and yet was also very different and fresh. 40 years on, E.T. for Atari 2600/VCS deserves a fresh reassessment without the problematic reactionary rejection of the hype of the 1982 holiday season. While no Atari adventure-puzzle game will achieve the well-deserved revered status of Robinett's "Adventure," "E.T." compares very favorably to similar classics of the platform like "Superman" and "Raiders of the Lost Ark." The fact that such a strong, complex and unique game was conceived, created, developed and delivered single-handledy by Warshaw in only a few weeks is an impressive testament to his phenomenal talent as a game designer and developer.