The difference is that online ratings are meaningless while OTB ratings aren't. I feel much the same about CC/Postal ratings as well........ at least post strong engine era...
Online rating and OTB what's the difference!
One is real chess and the other is fantasy chess--anyone who brags about an on-line rating is not a real chess player.

One is real chess and the other is fantasy chess--anyone who brags about an on-line rating is not a real chess player.
Both are real chess that's sure. Almost all chess players played online. Just because we didn't see face to face doesn't mean it's a dream. And online rating too probably has its benefits. Skill is required to play both online or otb.

The difference is that online ratings are meaningless while OTB ratings aren't. I feel much the same about CC/Postal ratings as well........ at least post strong engine era...
Not all use engines, the majority of the players play fair and square. Though officially recognized organisation like fide, uscf didn't give online ratings but still its good to determine one's strength where we stand etc.
IMO the only rating that counts is the one you get playing OTB chess, the rest do not mean a whole lot. I have been playing OTB for over 30 years and feel safe in saying that most of the people online with 2200 ratings for instance would not fair very well against a player with an OTB rating of 2200.

Online ratings are a little like the "work from home" part of Chess. You can have 100's of games without getting out of your pajamas, so they tend to be a bit more relaxed and casual. I would say that anyone who actually goes and plays in OTB tournaments, dealing with the travel, expense, nerves, etc., is going to be much stronger, point-per-point, than someone who plays entirely online.
By how much, I couldn't say, but a rule-of-thumb would be to take the percentiles and match them up.

My thoughts on it.
In my own experience and I strongly believe that it is the same with the majority of other players that there is not much inflation comparing online ratings and otb provided players play the game fair and square.
Because they're different pools of players, the ratings aren't the same everywhere. ICC and maybe also FICS have a rating survey you can add too by submitting your online and OTB ratings. It's not letting me view it right now, but I think 80% or so of players are within 200 points (some higher, some lower, and as much as 20% not even within 200 points).
For example, here, the top of the blitz rating pool will be 2200-2300, the top of ICC is 3000+ Yahoo! maybe 2100-2200, etc. Of course this isn't only at the top, but speaks of the players in the middle too that their ratings can't be 1:1 with other sites (although as you say the extremes are always less accurate).
I also compare my play against fide rated players who are 2000-2100 and the strength is more or less equal.
But their FIDE rating is for long time controls against serious players... hard to compare to throw away blitz games. You see yourself that your rating can vary depending on the time control (as it is for most people). I know for myself I play more attacking and gambit type games in online blitz that I'd never go for at a tournament. It's hard to believe you can compare your blitz play to someone and then believe it will hold true also for tournament standards.
2. Players who can play 2000 in blitz is sure to play better with more time like otb.
This makes it sound like you've never played in an OTB tournament. Long time controls are very different. If you're mind is used to blitz, your thinking can be too fast and unstructured for long time controls... the point in standard games is accuracy and doesn't have much to do with speed. Strong blitz play doesn't have much to do with a strong long time control game.
Also because the games are so much longer, you can fool yourself and over think, or get tired and actually make blunders you would never make in blitz. I've been there (and seen others do this) so I know.

Well!! As far I am concerned if I can beat someone who is 2000 in online rating I can also do the same to fide rated players who are 2000. After all chess is chess! And the rules are the same, queen is not stronger in otb(lol). But like you had said otb is more about accuracy rather than the speed, I agree with that. As a compensation we get more time to think. This more or less improves the accuracy a great deal. Let's take the example from top players, Players like Anand, Kramnik or Carlson can play against computers in tournament conditions but in blitz it clearly favors the engine.

Just because you can beat a 2600 online player does NOT mean you can beat a 2600 OTB player....... there is a WORLD of difference.

He must have meant "If I can beat someone rated 2000 FIDE in an online game, then I can beat someone 2000 FIDE OTB".
Not as egregious an error as "If I can beat someone rated 2000 online I can beat someone rated 2000 OTB", but still a bit suspect. Someone who is 2000 FIDE most likely takes their OTB games more seriously than their online play. Online, they may play while watching tv, or while their wife is nagging them.

He must have meant "If I can beat someone rated 2000 FIDE in an online game, then I can beat someone 2000 FIDE OTB".
Not as egregious an error as "If I can beat someone rated 2000 online I can beat someone rated 2000 OTB", but still a bit suspect. Someone who is 2000 FIDE most likely takes their OTB games more seriously than their online play. Online, they may play while watching tv, or while their wife is nagging them.
I think that he did indeed mean that being able to beat someone online rated 2000 means you can beat a FIDE 2000. This is suggested by him saying that there is not much inflation in online ratings. That claim, however, is simply false. We have 2800+ rated people here on chess.com that are not even titled. Now it's true that some of them cheat and use engines, but I refuse to believe that that is true in all cases.

I disagree with point 2 -
1) Blitz favors quick, shallow calculation. Classical time controls require deeper and wider calculation.
2) Blitz is much more forgiving of dubious openings and general lack of endgame knowledge.
3) Most of the people I know have higher blitz ratings than OTB ratings (and they aren't even close)
4) Some players can't maintain concentration for 6 hours, this inevitably causes them problems in classical chess.
I also disagree with your 3rd point. IMO the vast majority of players are weaker than their initial rating of 1200. This causes a ripple of inflation.

He must have meant "If I can beat someone rated 2000 FIDE in an online game, then I can beat someone 2000 FIDE OTB".
Not as egregious an error as "If I can beat someone rated 2000 online I can beat someone rated 2000 OTB", but still a bit suspect. Someone who is 2000 FIDE most likely takes their OTB games more seriously than their online play. Online, they may play while watching tv, or while their wife is nagging them.
I think that he did indeed mean that being able to beat someone online rated 2000 means you can beat a FIDE 2000. This is suggested by him saying that there is not much inflation in online ratings. That claim, however, is simply false. We have 2800+ rated people here on chess.com that are not even titled. Now it's true that some of them cheat and use engines, but I refuse to believe that that is true in all cases.
Like I have mentioned before inflation is likely to be on the extreme(high and low). 2800+ is a champion's rating and I don't think that the player with 2800+ in chess.com is a champion. The point is the medium is as accurate as ever. Also I don't agree with you that the 2800+ player in chess.com is not even titled.
"Now it's true that some of them cheat and use engines, but I refuse to believe that that is true in all cases."
Let's assume that the player does not use engine:- Either he is an unknown genius/rare talent or the known GM playing anonymous.
If he is using engine well in that case no one cares and so no title for him/her.

I am rated 1900 USCF and 2200 chess.com correspondence. That's pretty serious inflation there. Everytime this kind of thread pops up we see evidence of a lot of people similar to my situation.

I believe that the OP is talking about Live Chess ratings, not correspondence. Your Live ratings appear to be a little below your USCF.
I can grow apple trees; therefore I can grow orange trees. I can take photographs; therefore I can paint pictures.
Get real. You can only compare like with like. OTB is not like online chess. My rating on another site is around 2000. I would expect to lose to anyone rated 2000 FIDE. Unless I'm allowed to recycle my pieces.
In my own experience and I strongly believe that it is the same with the majority of other players that there is not much inflation comparing online ratings and otb provided players play the game fair and square. I have played in websites like chess.com, fics, letsplaychess.com, instantchess.com, yahoo etc. and my results are fairly constant. I also compare my play against fide rated players who are 2000-2100 and the strength is more or less equal.
Online Performance
websites --(Rating)Blitz-----lightning-------standard
Fics ---------------- 2013-----2012------------2014
chess.com-------- 1896-----2081-------------
Points:-
1. Given the same atmostphere players play accordingly say in online people play online and otb people play otb.(no cheating)
2. Players who can play 2000 in blitz is sure to play better with more time like otb.
3. There is a lot of inflation in the highest and lowest(3000-1400). 3000 in online doesn't mean players play better than Anand or Carlson so also players who are 1400 online doesn't mean that they play worse than 1400 fide.
4. Players in the medium determine their strength more accurately.
Other opinions are greatly appreciated because I believe that this is point of discussion for quite a long time.