A pretty unremarkable year, although it's worth noting that since the 70s reprint titles (Marvel Super Action, Marvel's Greatest Comics, etc) stopped prior to this point (for good reason, i suspect), i am missing a lot of the core titles for 1971. Gerry Conway starts to write some of these books, and they border on incomprehensible.

The most interesting thing that happened this year is Stan Lee's break from the Comics Code Authority to print some anti-drug books. This is not as daring as it may sound since the books were clearly anti-drugs, and he was doing so at the request of a government agency, but it was still a risk since a lot of stores at the time only carried code approved books. Marvel's ability to sell books without the code (for three issues) made the Authority question its relevance, and it revised a number of its rules later in the year.

First appearances: Jarella, Doc Samson, and Morbius.

Hulk #135  
Sub-Mariner #33  
Amazing Spider-Man #93  
Astonishing Tales #4-5  
Amazing Spider-Man #94  
Amazing Spider-Man #95  
Hulk #138  
Amazing Spider-Man #96-98 non-CCA approved anti-drug issues 
Hulk #139  
Captain America: Sentinel of Liberty #5-6  
Avengers #88  
Hulk #140 1st Jarella 
Astonishing Tales #6-8  
Daredevil #78  
Hulk #141 1st Doc Samson 
Amazing Spider-Man #99  
Hulk #142  
Fantastic Four #105-116  
Amazing Spider-Man #100-102 1st Morbius 
Sub-Mariner #42  
Daredevil #81  
Hulk #145-147  
Fantastic Four #117  
Amazing Spider-Man #103-104  
Marvel Feature #1 1st Defenders