My mage finally got his hands on the coveted [Mark of the War Prisoner] last night, putting me within reach of the "hit cap" for Wrath. With the Mark, my spell hit set provides 357 hit rating, which is 11 shy of the "hit cap" of 368. However, one of my spell hit items is the 29 rating [Cape of Seething Steam]. If I take off that cape and go with whatever I would normally wear instead, I'm exactly 40 shy, which is exactly what you get from the well-fed buff for eating [Snapper Extreme]. That does mean passing on a spell power buff, but it's probably worth it in the short term since it lets me keep more of my everyday gear on.
Reaching the "hit cap" was not a high priority for me, since Archavon is really the only raid content that I do. Now that I'm kind of here, though, I might as well take a look at the mechanic.
What is the Hit Cap?
WoWwiki has the basics on Spell Hit. With changes that came with the 3.0 patches, the maximum hit rate for spells is now 100% (it previously capped at 99%). It is very beneficial to reach this cap, because it means that you won't have spells failing to hit (in the process, failing to proc trinkets/talents, crit, messing with your threat output, etc). For my mage's current gear output, hit rating is slightly more valuable than even spell power.
The first catch is that there are actually THREE 100% marks to watch. Against other level 80 targets (e.g. players in PVP), reaching 100% requires 4% hit bonuses from gear, talents, buffs, etc. Against level 82 targets, the highest level you will encounter in 5-man content, that magic number is 6%. Against level 83 targets - raid bosses count as three levels above the player, which, in this case, means 83 - the number jumps up to 17%. (You may have noticed this while leveling - Blizzard seemingly does not want casters to fight mobs that are more than two levels above our heads on a regular basis.)
Almost all DPS caster specs have access to 3% spell hit via talents (Smite-Spam priests are out of luck, while Arcane/Frost mages are slated to get access to 6% hit in patch 3.0.8 due to a tweak to Elemental Precision), so those first two numbers are not all that hard to reach from hit rating that's on your other gear. The worst case, in which you've opted to skip your spell hit talents and need to get enough for 5-mans from gear, is only 158 hit rating, which is still easier than it sounds. So, really, the only "hit cap" that requires serious attention is the raid cap.
A further moving target
Once you move into a raid group, the numbers tweak again. If your raid has a Balance Druid for Improved Faerie Fire or a Shadow Priest for Misery, they will grant everyone in the raid 3% spell hit (the two do not stack). Additionally, Draenei grant members of their party (but, from what I've heard, NOT members of other parties in the raid) a 1% bonus from Heroic Presence (Draenei mages will grant this bonus to themselves).
So, you want 14% hit after talents, unless some combination of those other buffs push you down to 13%, 11%, or 10%. See why I've been using airquotes around the term "hit cap" yet?
So, what are the actual numbers?
The actual number for a caster with 3% hit from talents trying to reach the raid hit cap is 368 hit rating. The tension here is that the hit cap really is a cap - the server won't kick you off if you go above the cap, but it also won't give you extra credit for doing so. If my raid gets a Shadow Priest or Moonkin, that 3% bonus makes 79 points of hit rating temporarily moot. That nice [Mark of the War Prisoner] has 73 hit rating on it, so I'm basically getting no benefit from the passive part of that trinket anymore.
As a result, it may or may not be helpful for your hit set to allow you to shed 26 rating (to 342 when grouped with Draenei), 79 rating (to 289 with the Shadow Priest/Moonkin buff), or 105 rating (to 263 with both of the above) as needed.
(Incidentally, did I mention that NONE of this information is documented officially in-game? I guess anyone who raids is supposed to figure this stuff out by hand, and the rest of us are supposed to know on our own that there's no need to enchant gloves for hit rating if we're not raiding and already have the minimal hit levels needed for solo/5-man content.)
How am I getting there?
My hit rating is split between small amounts sprinkled on my everyday gear and large chunks on specialty hit gear. The breakdown:
[Flameheart Spell Scalpel] - 34 hit rating
[Ward of the Violet Citadel] - 38 hit rating
[Wand of Blinding Light] - 15 hit rating
[Heroes' Frostfire Gloves] - 33 hit rating
Boot Enchant: Icewalker - 12 hit rating
(Again, just my everyday gear puts me well over the top for 5-man stuff - I could spec out of two of the three points of Elemental Precision if I wanted to.)
Hit Rating Sidegrades:
[Water-Drenched Robe] - 68 hit rating w/ socket bonus (actually equal to the hit rating on the [Ebonweave Robe])
[Cape of Seething Steam] - 29 hit rating
[Mark of the War Prisoner] - 73 hit rating
[Rune of Infinite Power] - 55 hit rating
Right now, the sidegrades are all upgrades in situations where I'm not at the hit cap. That situation may not continue to hold up as I improve the gear in those slots (especially that green trinket, and my wand), and then I may have to start looking other alternatives (e.g. [Hateful Gladiator's Pendant of Ascendancy] to trade in the 34 haste rating on my Pendant of Subjugation for 34 hit rating) to compensate.
Why have a hit cap?
I guess the question I'm left with as I consider how to move forward is whether it's actually a good thing to aim players at a specific rating cap.
Casters aren't the group that encounters problems with arbitrary caps on stats we're otherwise encouraged to stack. Tanks encounter the same thing with the need for uncritable defense ratings. Back in the old patch 2.4 days, when I made a solo uncrushable Pally simply because it was possible to do so. The paradox was that defense rating was very valuable until I actually became both uncrittable and uncrushable, after which additional contributions to block rating became much less valuable. Then again, even in that case, the added defense rating was not totally wasted - it contributes to avoidance, and to block rating when Holy Shield is down. Hit rating is actually wasted when you have too much of it.
None of this would matter if we actually had direct control as players over how item budget is spent. Since we don't - perhaps in part so that Blizzard can add better items later - I don't think it's entirely unreasonable to be ask that large blocks of item stats don't suddenly stop working because something pushed you over the cap.
It may seem obvious that you can't hit more than 100% of the time, but couldn't excess hit above that cap spill over into something else, such as crit? Mages aren't going to deliberately stack excess hit rating to get more crit - what we actually want is spell power - so I don't really see how this would be abused, but it could always be taxed at some rate if needed (e.g. 50% of excess hit rating becomes crit rating). Regardless, having to juggle various sidegrades in order to continue to meet a static cap (be it hit, defense, etc) when you upgrade other portions of your gear or your party changes is a very clunky, meta-game heavy mechanic. I would definitely like to see some change on this front.