Just to fill in the expected argument...
The issue with mindsight is that, while it requires telepathy, and the Normal section implies that it uses telepathy, the actual benefit of the feat never says that is a form of telepathy, or that it is mind-affecting. It’s also not defined as a divination. Therefore, by the rules strictly as written, mindsight works, even if you have immunity to telepathy, to mind-affecting abilities, or to divination (e.g. you are undead or have mind blank).
This is because a feat’s requirement need not actually be related to how a feat functions (e.g. you don’t need to use Combat Expertise to benefit from Improved Trip), and the Normal section is explicitly treated as “reminder text,” to use a term from that other Wizards of the Coast product, and has no rules weight (e.g. even though the Normal section on Tower Shield Proficiency claims that nonproficiency results in taking the shield’s ACP as a penalty to all skills relating to movement, that’s not what actually happens because the “real rule,” given in the Armor section, is that all Strength- and Dexterity-based skill and ability checks take the penalty). Thus, there simply is no rule that mindsight is or relies on telepathy.
In practice, in all but the strictest games, this oversight it ignored, and lots of things protect against mindsight, such as immunity to telepathy, mind-affecting abilities, or divinations. Many also extend the Darkstalker feat to cover mindsight (and the lifesight feat from Libris Mortis). As Sandwich pointed out in a previous version of his answer, the Madness ability of allips would also form a serious deterrent to the mindseer when Mindsight is treated as telepathy, as the mindseer would quickly lose all Wisdom.
Even under the rules read as strictly as possible, there are still some defenses against Mindsight.
Mindless creatures are immune to Mindsight. Not terribly helpful for players since you can’t play a mindless creature, but it’s worth keeping in mind, particularly as a DM. Note that intelligent creatures that are usually immune to mind-affecting things, like intelligent undead, are not immune under this super-strict RAW.
If the mindseer is put inside a hellbreaker’s Telepathic Static, as Matthieu M. discovered, it loses telepathy, and can no longer use the Mindsight feat as it has lost the requirement.
As Forrestfire just pointed out to me, when you have Total Cover (as when employing a tower shield), creatures cannot draw line of effect to you. Normally, this doesn’t matter much since touch attacks and targeted spells still work against the tower-shield user, but Mindsight is not targeted, so the shield somehow blocks the Mindsight, despite being far thinner than the requisite blocking material. This is really just an abuse of the terrible wording on that use of the tower shield, more than anything else.
As Sandwich found, the Cerebral Blind class feature of the slayer class can be read as blocking all effects that reveal location. Note that the wording is somewhat ambiguous.