Mainly problem is talent points (TP) and experience points (XP). Whole game without dailies has enough only for 1 class to level to cap. Doing dailies every day, if possible highest at your level will off load that and make it possible to level secondary class as well.
So far - everything OK.
As you reach cap, you start to put TP to skills that you find useful for your class combo and this is where fun starts. You need around million TP for each level (bit more then million as you come close to skill cap) and you can farm that in 4-5 days of most used dailiy for level cap (Inferno Butterflies). Add to that other class combo, all elite skills plus other skills that you can find useful... and you will need a year to cap everything, and in mean time game will have higher level caps, thus more TP required... (see where we going with this)
Now, that is just part of the problem. Other problem is that it is fairly expensive to gear one class. M/P and P/M might seem similar, but if you like to be either high end DPS or high end healer, you will have to stat and gear differently. If you between... well then you will not be either of those things that you like to be.
Hope this helps.
