Marvel's Iron Man knows the Avengers' biggest weakness...and unfortunately, it's one that can't easily be eliminated. While not the leader of the team, Iron Man often bankrolls the group and supplies much of the equipment and training facilities. It is only natural, then, that he also considers all methods of eliminating weaknesses, but in Avengers #29, he realizes the Avengers will always lose against a group like the X-Men for one horrifying reason.
The Avengers are an eclectic bunch of superheroes, to say the least. The team is a mixture of ordinary humans with no powers (Hawkeye and Black Widow), super-soldiers heightened strength (Captain America) and even characters who can fly through space and possess unimaginable power (Captain Marvel and Thor). But while the higher-powered characters can punch through mountains and go toe-to-toe with the likes of Thanos and Doctor Doom, they have the same weaknesses as every other Avenger: none of the members are expert telepaths.
In Avengers #29, written by Brian Michael Bendis with art by Walter Simonson, the Avengers realize they're at a significant disadvantage in their conflict with the X-Men (this was during the days of the AvX crossover). "All the best ones are mutants," Iron Man says, when the other Avengers realize they need a telepath to bolster their numbers. "We won't be able to trust them." Stark is right, as he pulls up a list of telepaths - almost all of them are mutants. Emma Frost, Mr. Sinister, and of course the leader Professor Charles Xavier are incredibly powerful telepaths - and the Avengers have absolutely no defense against them.
The Best Telepaths Are All Mutants - And The Avengers Have None
In the Marvel Universe, a single telepath, with enough training, can devastate an entire group of soldiers, even Avengers. While some characters are resistant to weak telepathy (the Asgardian Thor, for example) and while it is possible for normal humans to resist the mental powers of a mutant, it is impossible to resist forever. Moondragon remains one of precious few Avengers who has mental abilities on part with high-level telepaths, but she can't withstand Xavier or his most powerful students.
To make matters worse, many of the X-Men's telepaths are not simply telepaths - they have other powers as well. Emma Frost can transform her entire body into a diamond form, Jean Grey has telekinetic powers, and Exodus has an entire suite of powers, each of which can devastate the classic Avengers roster. Iron Man, for all of his faults, is right: in a battle between the Avengers and the X-Men, the mutants of the Marvel Universe would win, simply by exploiting the Avengers' biggest weakness.