To clarify the answer for the PvP side of the original poster's question, you'll have four things working against you:
1) Stats don't scale toward the bottom of the 29 bracket the same way they scale toward the bottom of the 19 bracket i.e. the difference between having 15 resilience at 20 isn't much different compared to 29, but 10 and 19 show a huge difference with 15 resilience.
2) 29s benefit from some really nice tier 2 talents, and often score with a great tier 3 talent using the final talent point you receive at 29.
3) Level 20 is no longer one of the major "ability plateaus" where you learn a couple of new, awesome abilities for your class. Before 4.0.1, levels 20, 40, and 60 all had major updates. While that's still true for a few classes, most classes saw those plateaus go away.
4) You learn your first set of glyphs at 25.
That said, you may want to search through class abilities and talent trees to see what's available. 4.0.1 brought a lot of changes to the table, and invariably someone will surprise us with an interesting find, so don't be quick to shut down unusual possibilities. As an example, there is serious talk going on right now about the viability of a rogue FC in 29s. While rogue damage takes the spotlight right now, some new abilities and gear showing up in 29s could make a slippery rogue into a great FC. If that sounds ridiculous, remember that it wasn't too long ago that a good mage FC could make a serious impact on 29s.
Have fun!
