League-of-legends – What does the AP Mid GP10 meta entail

It should first be said that the metagame always has the ability to change at any given time. Several factors impact this heavily, namely success found in tournaments by the competitive professional teams.

As seen at IEM Hannover, the meta game between the three regions is essentially the same. All of them utilize AP mid, ranged AD with support bot, jungler, and either a bruiser top or AP, depending on what team comp they were going for and counterpicking. This has effectively been the metagame in all regions, and until someone comes with a different metagame that does well against it in a tournament setting, it will likely stay.

Prior to Dreamhack, the NA meta (can't say US only, since many players from Canada play, for instance of several members of CLG and TSM are Canadian citizens - at WCG: ChaoX, TheOddOne, HotshotGG, BigFatLP, and Elementz represented Canada) ran quite differently to EU. NA teams at the competitive level ran a tank top, jungle, AP mid, ranged and support bot, while EU teams ran double AP in solo lanes, jungle, ranged and support bot. The casual games on NA was still running, double tank lanes bot, AD carry mid, AP top with jungle, and many games had no jungler. I know this because this is when I started playing LoL heavily. At Dreamhack, it showed that EU was more used to AD with support bot, and great plays were made continually by their bot lanes. The eventual winner of course was fNatic led by xPeke and Shuushei showing off their double AP comp to quickly burst the enemy team with AoE damage and making it easy pickings for their AD to finish everything off.

Flash forward to IEM GamesCom, the NA teams now having chance to get more practice with the AD/support bot lanes and think of ways to counter the double AP strategy. NA teams, come up with tanky DPS top and tanky DPS jungle giving way to the Bruiser meta. In one game of TSM against an EU team I cannot remember, TSM takes Soraka/Jax bot lane to combat the Urgot who had given them problems the game before. They had three bruisers and managed to pull off the win before falling to CLG in the finals where the first two games was playing protect the KogMaw.

To answer your question then, it wasn't really a matter of teams playing different metagames, however they understand what counters what and know how to play accordingly. Double AP gets countered by a bruiser/tanky DPS comp. AoE team comps can win teamfights but can be slaughtered to a poke comp where they don't allow a teamfight to happen before they poke you down too much, a push comp counters a poke comp. So counterpicking, and strategies to execute is used a lot to help gain the advantage. For instance, picking a very strong late game comp against an early game team comp and playing super passive to stall the game. Nasus, Irelia, Karthus, Vayne, KogMaw are examples of champions that have an insane lategame but an early game that can be shut down (not so much in the case of pre-nerf Irelia). On the other hand, Urgot, Ezreal, LeBlanc, Alistar, Udyr start to fall off heavily in the late game compared to their counterparts unless they have built up a big item advantage.

As for everyone knowing and following the meta, it can be both a benefit and negative depending on what side you take. In many tournaments, teams have picked like they were following the meta, but in game swap lanes to give themselves the advantage. This is most common with top and bot lanes switching. It also happens with mid lane and top lane as most players use their runes to give themselves a laning advantage. For example, let's say one team's top and mid are Malphite and Galio respectively. The enemy uses Riven and Karthus. Malphite thinking they will face Riven, choose to use a full armor page to effectively negate most of Riven's harass combined with Malphite's passive. Galio similarly use full magic resist page to negate anything Karthus can do in lane. Lane starts and you find they swapped on you. The difference is they could have chosen to rune for it or not, but either way, Malphite and Galio is at a disadvantage since their runes do little for them. Sure Malphite's damage builds with armor and Galio gets more AP with magic resist, but they aren't going to have much fun in the lanes. To compensate, they try to quickly switch. But the enemy team can switch as well, forcing another switch or unfavorable matchup. Each time you switch lanes though, one of your lanes will tend to fall behind, and since you made the first switch, your team will lose out allowing the other team to gain an advantage.

[Updated for Season 3]

The game is currently in flux (season 3 has only been out for a couple days), but with the boosted ambient gold gain, it appears you only need one gp/10. I've been building Philo Stone to great success. If you expect to get some significant assist-gold (you're confident in your kill lane, or you're Soraka and have a good eye for your ultimate), you could probably stand to buy a Kage's Lucky Pick, but I much prefer to only buy the one Philo Stone and start immediately after into my build (Sightstone, Shurelya's, Aegis -> Bulwark).

Again though, this is all pretty fresh. Your mileage may vary.

[NOTE: The answer below was provided for season 2. There were significant changes implemented for Season 3, and this answer is now out of date. It is provided here for history, but should not be followed.]

It depends. The rule of thumb is two (Philo Stone and Heart of Gold), and 98.355% of the games, you'll want those. If the game is going to absolute crap, teamfights started early and your team is getting routed, you may have to skip the second and start pouring that extra money into wards and an oracle's (counter to what you'd think, oracle's is a great purchase if you're falling behind and the laning phase is over).

I'd be very suspicious of my support if they got a third gp/5. The only real time I can see that being worthwhile is if you know it'll be a late, late game, and that you'll need to be a significant contributor in teamfights for damage.