My experience with my 5 shaman in leveling from 60-80:

TBC - I quested and used quest green gear till I over-leveled instances by a level or two and then I would do the instances. Its nice as a change of pace and you get the instance quests done which nets you good gear and xp. TBC instances are pretty easy for the most part with the last boss in Ramps probably the most difficult in all the 60-69 instances at the correct level.
I made 70 before I even finished Nagrand I think, so thats Hellfire, Zangramarsh, Terrokar and Nagrand but I didn't skip quests.

WotLK - I found the first instance much harder than the TBC instances. The mobs just seemed to hit much much harder but this was at 71. I just decided to skip the instances and leveled with quests and AV. Made 80 halfway through Grizzly Hills without skipping any quests.

Yes, your gear is going to be crap at 80. If you have the gold you can get a pretty good crafted blue set and if you did all the quests you will have enough rep to buy a decent weapon and shield. Also, if you did a lot of AV then you should be able to afford at least one very good PvP item.