OP, thanks for posting this. Good to know I am not the only one. I'm a winning live player (~+6k) and an online fish (~-3k). I have been contemplating this question for a long time and here are the things I have realized so far:
a) Playing the same stakes:
I used to play 1/2 NL live and therefore I played 1/2 NL online ... BIG mistake. I got OWNED badly time after time. As so many others have pointed out, online play @ 1/2 NL is way more stronger. I have now started playing 0.05/0.1 online and while I still get owned, I don't get owned as often
b) Distractions:
In live poker, you are not that distracted even if you are not in the hand since there are real people you can talk to and look at. When people are thinking, there is still something going on for you to look at. With online poker, at least for me, I found that I grew impatient when I was not in a hand because it was quiet and all I was doing was staring at my computer screen and watching a timer run down. I would try and bluff often and in spots I never really would because I was bored. Really, I found online poker incredibly boring at one point in time. I have now started playing 6-tables at once. My impatience has vanished. I am now usually always in a hand and can fold garbage pre-flop much easier. Besides, its new, challenging and invigorating. If you have never tried this before, believe me, @10 NL its very easy and keeps your mind occupied.
c) Table image:
In live poker, people 'read' off your real life personality. I'm a cheerful, bespectacled, studious looking guy with a soft voice. People invariably read this as reliable and trustworthy. I am allowed to bluff often without anyone at the table suspecting a thing! Try that online
d) Table Flux:
In live poker, you will see new people joining your table *may be* once an hour. The tempo of the game does not change that fast. You see way more people entering and exiting your table in online poker. The tempo changes a lot more. I have re-evaluated some of my live strategies to adjust to this fact. E.g: Don't play (say) super-loose in the beginning just so that people will call your monsters down with nothing later on. I found that playing tight through and through online works
e) Tracking software:
Yep, people use software that tracks you to aid their decisions. Of course, in live poker, many players don't keep score. If your opponent has no idea of how many hands you played and in what fashion, chances are they will not take the right decision.
There are some other factors like casinos having richer recreational players, the strength of online players depending on the time of the day you play, etc. but I don't yet know enough to comment on those.
I hope more people who have successfully made this transition will reply to your question. Good luck!