Everyone here is likely to have a personal opinion on how easy or how difficult it is to solo in this game...
As for me, I tend to solo a lot because I like to! I play both a lvl 23 ranger and a lvl 23 warden. With either, I can solo white and yellow targets with no trouble. Orange and red mobs get a little dicey but are doable. So far, over the course of this 3 day weekend I have already made 2.5 levels with my ranger, (started at 21 on Friday night), and it's only Sunday night!!!
I think that how successful you are at soloing is going to be dependent on:
A) how well you play your toon
B) what class you choose to solo with
C) how well geared you are &
D) what your expectations are
About the last, if you expect to take on epic targets that are 10 levels above you... well it's probably not going to happen for you! On the other hand, if you go after mobs that are your equal or less, I think you'll find that this is a very soloable game.