But OP is at level 51, where he'll need 1000 xp for a level or 1000 xp
for an AA.  We'll assume that due to his low level it takes him twice as
long to get 1000 xp as it would at your hypothetical level.

Thus, at 50%, it still takes 4 hours for him to get both... but at 100%
AA he'll get that first AA, and then perhaps he'll be able to use that
AA to get XP 10% faster (whether because it helps him that much, or just
because he doesn't die with it as much so it saves him that much lost
XP); now it will only take him 108 minutes to level.

Or he could put 100% into real xp, and level in two hours, when he'll
probably get xp half again as fast, so it will only take him 80 minutes
to get the AA... unless he leaves off AA again and goes for 53 instead,
which will still take 2 hours (because he needs more xp to level to 53);
now he'll be able to get that AA in 60 minutes due to leveling faster.

This is why people end up waiting on AA; because the amount of effort
needed to get an AA drops as your level goes up.

The only problem with this theory is that some AA are really worth the
extra effort needed to get them; I'd count Run amongst those for many
classes, as its cheap, has no prerequisites, and represents a paradigm
shift; now you are faster than almost every mob without worrying about
whether a buff has run out or been dispelled; running away or kiting a
mob around till its under control become constantly available options.

I'm not sure I'd count any other AA as worthwhile in this way for most
classes; pure melee might find the regen one handy, or the one that
improves Bind Wounds, if they solo a lot.  Barring that, the rest of the
ones you can get under level 61 all require you spend those three extra
points (the ones above and beyond Run 3) plus most require several AA
points per ability.  That means to get one ability may cost you 6 AA;
the equivalent of leveling from 51 to 52 6 times over... not many are
really going to change your character as much as simply leveling would have.

The other trouble with this theory is that for many classes, leveling up
may represent a drop in xp gathering rather than a raise; if you solo a
lot, you'll find that the mobs get harder faster than your power
increases.  This is a deliberate game design since the early Verant
days; we were supposed to start out solo, then get a partner, then get a
small group, then a full group, and finally end up raiding; in this way
our power would increase exponentially, so mob power levels also
increased exponentially.  Sadly, a single player's power tends to
increase only geometrically.  Worse, caps to effectiveness of stats or
spells means in some cases player power actually plateaus out; there are
limits to what a bard can charm, for example, so his best xp gathering
method finds his targets being less xp each time he levels up.

If you group regularly and never have trouble finding a group, then that
problem shouldn't apply at all, in fact I've generally found with a good
group that the time per level actually drops each time you level up (and
the rest of the group does), because of the synergistic effect of the
increased power of its components.

Grage

However:  The cost of an AA is constant.  The amount of XP you can
get per hour goes up as you level.  Therefore, in most cases, getting
to 65 first then AA'ing will get you the to the goal (65+X aa's) faster.

The death takes 100% of its penalty from regular xp, always. It ignores
lxp, and aaxp completely.

As to losing AAs? No. It doesn't happen. However, I believe you lose the
ability to -use- a completed AA until you re-ding if you lost a level,
and the AA requires you have that level. I can't recall for sure... but
it would seem an exploit to allow you to unding back down to level 20 on
a pvp server but let you keep your 400 AAs...

You never lose any AA exp or any AA, although if you lose a level that
was required to buy a given AA you won't be able to benefit from that AA
till you regain the level.

When you get a rez, all the exp restored to you goes back to your "real"
exp bar as well, even if you thought you'd like to go ahead and dump
some deaths and put 96% of the exp into an AA for some reason.

Many people put 90% exp into AA instead of 100%, in order to make up for
deaths.  A more common practice, however, is simply to build a "cushion"
of about 2 blue bubbles into your next level before switching to AA, if
you switch back and forth.

Grage

Well, kinda.  You can get +2 mana regen starting at level 8 or you can
group with a 16 enchanter (before you hit level 20) and get a buff
that is mildly better (since it has no hp loss attached to it).  Then
you can group with a 29 chanter and get Clarity, which also provide
the same mana regen as what you already had (at level 20)...meaning it
is only better because you dont lose hitpoints.

In any case, I never bothered through these levels.  I soloed them,
for the most part, each in a few hours anyway. =)

Marginally superior, if at all...since you gotta group with the
enchanter and count on him or her to keep casting it on you.

This is pointless.  These buffs last 30 minutes.  You lose more
experience running around for short-duration buffs than you would
allegedly gain by not having to lifetap once every few minutes.

This I forgot about.  I do remember keeping C3 a few times in my late
40's, but still never actually paying for it.