I think it's his lack of guard passing skills.
Both defensive and offensive, and it lost him his last two fights.
In the Werdum fight fedor had an opportunity to pass werdums guard after he escaped the first triangle attempt but he seemed completely oblivious to it.
Instead he allowed werdum to reset his guard and go for the second triangle which he locked in.
In the silva fight Fedor jumped right into silva's half guard twice during the first round and never even tried to pass the guard and secure side or full mount where his sub attempts or ground and pound would have been much more effective.
Then in the second round you saw how silva got into Fedor's half guard and was able to pass it very easily into side and then full mount where he unleashed his onslaught of ground and pound that eventually ended the fight due to the swollen eye Fedor suffered.
This is a weakness Fedor has shown throughout his career, I have never seen Fedor successfully pass anyone's guard, heck I've never even seen him try it much. He just seems completely unconcerned with it.
And his own guard has been passed quite easily by alot of his opponents, Nog, Arona, Coleman, Herring and even mark freaking hunt were all able to pass his guard.
In passed fights this has not cost him because he has always been able to scramble or power out of the bad positions he gets into, but it ultimately caught up with him in his last two fights against BJJ black belts who were alot bigger and just as strong as him.
Fedor really needs to pay attention to this part of his ground game and fix it going forward. I'm pretty sure other fighters are well aware of this weakness now and will look to exploit it.
I was suprised Fedor went to half mount against Silva. I actually yelled at the TV "get in his guard!!!" He got swept from that position a couple times against Nog and was the only time he really got himself in trouble in those fights. Silva's a big mofo and very good at sweeping from that position, especially since Fedor does not have the weight to keep him down.
Fedor has always been happy staying in guard and pounding away. He got caught be Werdum and I'm sure that was in his mind, but IMO that was the safest position for him.
Beastro -Fedor's biggest weakness?
Ice cream
Lol. I think you are onto something...
Or Sambo. For a full ground game, rock solid and the best in the sport, he needs BJJ. Each covers about 1/2 the range of the ground, one upper, one lower. If he becomes a BB in BJJ, he would dominate how the ground worked in totality. He is an excellent striker and has a knack for getting hit over and over again, then comin' back for a win.
I think if Fedor learned BJJ, the rest would be small diet/training/conditioning adjustments to make him the best p4p fighter.
Being awesome. 
Fedor's biggest weakness is that he cant beat his hand picked opponents anymore.
Yoshida4Life - Fedor's biggest weakness right now is that he has lost the fire that made his career so exciting during the Pride era.
He was talking about retirement even before the Werdum fight. That means his heart is no longer in it.
So much this.
So many Fedor haters are using his recent performances to downgrade Fedors past opponents and talk up the current heavies like they are just so much passed Fedor skillwise.
If you guys can't see that Fedor isn't the same fighter he was a few years ago, you're crazy!
But, OP makes good points though about the guard passing. Fedor was always able to power out before, and he's just not strong enough anymore. he's lost SO much of his muscle mass and functional strength.
Watch his old fights, like the 3rd Nog fight and a couple of the takedowns in the Herring fight, Fedor was fucking unbelievably strong and fast. It was amazing to watch, it still freaks me the fuck out every time I watch those fights.
"swats Fedor's ballsack away from face"
lol, at least I can admit it, right?
^ Q1= No one Q2= No one ...his safest position against Werdum was standing up.. he wins that fight if he tells him to get up.. Silva? just too big.. he had no answer for the mount except his usual explosive bumping which failed against a 50-60 lb heavier black belt.. trying for an elbow escape at least a few times might have saved his eye and at least been a better option than giving up the back
Fighting at HW as a fat MW
crazydave -this... ^^^
hitman8 - I think it's his lack of guard passing skills.
Both defensive and offensive, and it lost him his last two fights.
In the Werdum fight fedor had an opportunity to pass werdums guard after he escaped the first triangle attempt but he seemed completely oblivious to it.
Instead he allowed werdum to reset his guard and go for the second triangle which he locked in.
I think Fedor, at least before the Werdum fight, was not scared of being in the guard. He was comfortable ground and pounding guys from inside his opponent's guard.
He doesn't care about being the best. If he did he would drop to LHW and train with guys that will make him better.
groundfighter2000 -hitman8 - I think it's his lack of guard passing skills.
Both defensive and offensive, and it lost him his last two fights.
In the Werdum fight fedor had an opportunity to pass werdums guard after he escaped the first triangle attempt but he seemed completely oblivious to it.
Instead he allowed werdum to reset his guard and go for the second triangle which he locked in.
I think Fedor, at least before the Werdum fight, was not scared of being in the guard. He was comfortable ground and pounding guys from inside his opponent's guard.
Clear case of overconfidence.
Fedor would smash Werdum 9 times out of 10 and if they ever fight again I don't think the fight will be competitive.
I personally think Fedor is still in the top 2 or 3 HW's. How many guys out there actually have the unique combination of skills and attributes that Big Foot has?
Is there anyone in the UFC that does? No.
Brock has a very suspect chin and absolutely no cardio.
Shane also suffers from a suspect gas tank.
JDS isn't a giant HW and would probably struggle to strike with Fedor who has easily the fastest hands in the division and probably couldn't hold him down the way Big Foot did.
Cain also has a suspect chin (see Kongo). What Cain does have is an extremely swarmy ground control game that might allow him to wear Fedor out, or simply decision him.
The only two guys I think might reliably beat Fedor right now are Big Foot and Ubereem. 'Reem cause he is just a fucking truck and I don't think Fedor could take him down, nor do I think he could expect to win a striking match.
Big Foot because he stands up well to Fedor striking, primarily because he has a giant fucking head and a dump truck for a chin, literally. Additionally he is a legit BJJ black belt and when 290ish pounds is sitting on your chest you're going to be hard pressed to buck that off.
The people that surround him are his biggest weakness.
I'm convinced he threw those fights to dodge Reem.
I'm a Fedor nuthugger, but I know he's a tiny bit shook to fight reem.
No way he can be making these rookie mistaks after 10 years of fighting.
It's pretty much impossible to stay undefeated for a long period of time, especially in MMA. Not only are there so many ways to lose, but when you have every other fighter studying your game and trying to figure out a way to beat you, eventually some fighter/team will figure it out.
He's never really been concerned with the guard b/c he hurts people with punches from there. I'd be confident too if I beat down Big Nog from the guard in 3 different fights. I think he expected the same fight out of Werdum, thought Werdum was really hurt when he went down from punches, and got caught by a sweet triangle.
His inactivity has hurt him over the last few years, and his size is also a factor now. He's giving up lots of weight and power to skilled fighters now, and was clearly the smallest guy in the SF tournament. He's become a headhunter with punches (kinda like Liddell) instead of mixing it up like he used to, and people can game plan better now.
If Bigfoot gets mount, have fun trying to get him off while eating those hammerfists. I don't think many people could. Anyone who's trained knows how heavy good BJJ guys can feel.... how about someone who weighs 285 sitting on your chest? Not a fun time.