It's not that there isn't more things to do-- there's tons of quests to do after you hit 70, lots and lots of rep to gain, plenty of great PvP, and plenty of instances and raids to run. It's the humbling realization that as much emphasis as both Blizzard and other players place on arriving at 70, it's hardly anywhere near the end of the game. Your level numbers stop rising, but there's still a lot of progression to go.
For some players (somewhat like spellpower, as it sounds like she's finding herself in a bit of a hole), it's discouraging realizing that there's so much more to do. I find it encouraging to hit 70-- while your gear isn't that great, you can finally run all the instances you want to run, and even start appearing in raids, even if you don't hit the top of any DPS charts. But no matter how you react, it is definitely a feeling that is unique to reaching "the top" of any MMORPG game. For the first time, you might realize that these games really never do end.