Consider this:

Level | Exp Needed to level
70 1,523,800
71 1,539,600
72 1,555,700
73 1,571,800
74 1,587,900
75 1,604,200
76 1,620,700
77 1,637,400
78 1,653,900
79 1,670,800
Tot 15,965,800

Compared to TBC

Level | Exp needed to Level
60 290,000
61 317,000
62 349,000
63 386,000
64 428,000
65 475,000
66 527,000
67 585,000
68 648,000
69 717,000
Tot 4,722,000

Taking in to account that mobs are worth between 850-1050 Exp ( 70 - 80 ) and quests are worth 22,500-31,625 You need to do about 725 quests with zero instances to level. If you are able to do instances it makes this number drop ALOT.

In fact when you are able to do Gun'Drak, if you are quick it takes about 15-20 mins and is worth 7.1 % of a level ( Level 76 ) So if you are able to do this over and over and over it's much quicker then questing.

Leveling my main ( warlock ) completing every zone up to IC and SP ( and one run of the instances, just for quests ) I am 78 - 7/10ths. So i can see that you really need to do just about every quest in the zone to ding 80 in SP or IC.