The background to this discussion is two Cardrunners videos (Brian Townsend and Fabian), Sklanskys "NLHTAP" and my own stats/reflections.
I find that aggression is emphasized more by online pro's (like Brian and Fabian here) than by live pro's and 'teachers' (like Sklansky).
Basically when you watch the OP (online pros) play they will pretty much always raise when first in, and many will 3bet light both in and out of position. Sklansky on the other hand recommends a more cautious strategy. For example, in NLHTAP he says it is sometimes correct to open-limp on the button. I doubt you will find more than a handful, in any, of open-limps on the button in a 100k database of most OP's. Basically their different strategies can be simplified like this:
Online pros:
If I raise I will often pick up the blinds.
If I get called, I will have position and my opponent is unlikely to hit the flop, so I can take it down with a CB.
If I get called, I can either make or have a hand, or get another opportunity to bluff profitably/outplay him with position.
By raising and re-raising light I will get more action on my big hands.
You should raise or re-raise the same amount whether you have AA or 76s.
But basically, they're looking to pick up a lot of small pots when their opponents doesn't have anything.
Sklansky:
In No-Limit blinds are not as important as in Limit, since they represent a much smaller fraction of your expected won pots. Therefore, raising should not be done to steal pots but rather (mainly) for value. It's OK to deviate from this time to time to add deception, but most of your raises should be based on the value on your hand.
Your preflop play should be adjusted to suit the way your hand plays post flop in relation to stack sizes and hand character. With a drawing hand you want to get in cheap and make money post flop, with a big pair you want to make it expensive preflop to not let opponents draw profitably against you.
To use a specific hand as example...
[5s]
With a few limpers, OP will raise to either win the pot immediatly or set up for a profitable CB.
With no limpers OP will raise to take the blinds or set up for a profitable CB.
With a raise, OP will usually either fold or re-raise, depending on opponent. With a few overcallers there is also the option to call.
3-betting is done with a wide range of hands. Usually with big hands (like AK or JJ), speculative hands (like 65s) or junk. The only hands they don't like to 3-bet is hands that will have superior calling value (AQ or KJs?) against the range, basically they're too good to fold but not good OR bad enough to 3-bet.
With a few limpers, Sklansky will limp behind, hoping to hit a big hand or draw.
With no limpers Sklansky will either raise, since his hand expects to be ahead of random blind hands, or limp to encourage worse players to play.
With a raise infront Sklansky will almost invariably fold.
3-betting is mainly done for value with big hands, and sometimes with junk that there would be no value calling with.
So you could say that OP's pay a BIG price to get a BIG chance at winning a SMALL pot. Sklansky wants to pay a SMALL price to get a SMALL chance at winning a BIG pot. Obviously it is hard to say what strategy is correct without actually knowing the exact (or aproximate) values of "BIG" and "SMALL".
The OP preflop stats are something between 20-30 vpip, and 15-20 pfr. I'd estimate Sklanskys numbers to be slightly tighter but also a lot less aggressive. Maybe 25/8 or something (my own stats are more like 18/14).
Now if we discount "meta game value" and "table image" and assume we are playing in a vacuum, but with reads on our opponents. And we are not playing beyond level 2. That means, we are only concerned with OUR cards and our opponents cards, but may also consider what our hand looks like to our opponent (in a vacuum). What do we think is a good strategy? What sort of aggression will have the highest long time EV?
**disclaimer**
Obviously these concepts are more complicated than I have portrayed them here but in order to be able to have a reasonably readable debate and not get too far off topic I have decided to boil them down to more manageble "bullet points".
Ps. If you don't like Brian or think Sklansky is a clown, that's fine, but keep it to yourself. If you want to discuss this, pretend they are the opinions of someone you do like or respect.