As has already been pointed out, "deep" in warm tropical waters is vastly different than "deep" in cold North American (or UK) waters.
Factors to consider:
1) - vis
2) - temperature
3) - exposure protection
4) - general equipment
5) - narcosis
1) Pretty self explanatory. Except a free ascent from 70' in Tobermory is quite different that the same in Cozumel. In the latter you can almost always see the surface, even at 110-120'. Not so in the former, where you usually wind up using a light after about 60' or so. Seeing is very important to a lot of divers. Some get panicky & lose control. Especially when they are in midwater and have no visual point of reference.
2) Deeper is colder. Again, not neccesarily a big deal, unless you are unprepared for it. That's also why most tables recognize the physiological impact this has on divers. There are rarely noticeable thermoclines in tropical waters while they are the norm in fresh water. For example, a surface water temperature of 77F can wind up being 54'F at 65'.
3) This is obviously tied to #2. It tends to be a somewhat subjective item. However, most people agree that you don't need much in the tropics, even at depth. Not so freshwater. And of course the amount or type of exposure protection can impact your mobility.
Also, what about the fact that neoprene compresses at depth?
4) Also linked to #2. Not too many people using drysuits in the tropics. Or cold water regulators. It also has an impact on the type of BC some people choose.
Also, what about redundant air supplies? If you are routinely diving deep, what type of redundant air supply to carry? And how?
There is also whether or not the regulator is balanced: an unbalanced reg is much harder to breathe from at depth.
5) Narcosis doesn't just manifest itself as "rapture of the deep" or "inappropriate behaviour". It sometimes shows up as an increased level of stress or anxiety in the diver. People who are not experienced in diving deep may not recognize ANY of these symptoms. Which could have some serious consequences.
Conclusion: there is LOTS to learn about diving deep; I've only touched on some of the main points. Doing your first dives with a qualified Instructor is one of the best ways to start learning.
~SubMariner~