Back to 2D, best move Capcom could've made at this point. Plethora of extras give this game a ton of replay value.
The first thing anyone who played X7 should notice is that while this game has 3D graphics, it is entirely in the 2D perspective, with the sole exception of the two ride chaser stages. This means the total elimination of the lock on feature from X7. You might be thinking "Wait, doesn't that make Axl worse than before?" but worry not, for Axl is actually better than ever. Now Axl Bullets are a rapid fire weapon that can be aimed in 8 directions, like Bass from Megaman & Bass, except Axl doesn't move when you try to aim diagonally. In addition to that, Axl automatically hovers if you start firing while in air, and with the new White Axl (equivalent to Black Zero or Ultimate Armor) he will indefinately hover while firing. But even with normal Axl, he can stay latched in place to a wall forever while firing (like Gaea/Shadow X from X5 & X6). Also the Copy Shot no longer requires to be charged which is definately a good thing since it was horribly weak in X7, it also seems to be a great deal stronger in X8. Speaking of the Copy Shot, you can now use DNA data whenever you want within the confines of the stage it was obtained in, rather than it being automatically used. It still does have a limit to how long you can use it, but the White Axl armor eliminates this limit in your second game (or possibly much later depending on how many games down the road it takes you to get 100% upgrades and the last hit on the final boss with Axl). Axl now has his own set of special weapons, all of which have infinite ammo. In most cases he will get an entirely new weapon, like a flame thrower or a gatling gun, but other weapons will be his normal pistol shooting different ammo. White Axl also adds a blue bullet into his shots, which I'm not sure of the purpose, but I assume it either does more damage or just breaks barriers. White Axl also has an EXTREMELY long ground dash, I haven't tested the full limits, but I have dashed all the way across from wall to wall in some of the boss stages with no problem in a single dash. Axl is a unique character now and is pretty useful now yay!
The idea of switching weapons with Zero was a good idea in X7. In fact it was so good that it was brought to X8 and expanded upon. Now you can purchase 5 new weapons for Zero: The Glaive (much longer than in X7 and faster), the Hammer, Fan, Knuckle, and after you beat the game once, the Sigma Blade becomes available. The Sigma Blade is an incredibly powerful weapon, it is almost as long as the D Glaive, it breaks through barriers, and it is the strongest of Zero's weapons. Also, every weapon can use any of the special attacks, unlike in X7. If the Sigma Blade is the last upgrade you purchase for Zero (i.e. beat the game with 95% for Zero then purchase Sigma Blade to get to 100% in the new game.) then you will automatically receive Black Zero, which, when selected, doubles his attack power and halves his defense. He is an unstoppable machine of destruction with the Sigma Blade and Black Zero. Also, Zero seems to have hyper dash in this game. He dashes quickly but short, giving him the shortest dash of all the characters. Zero has perhaps the only gripe I have with this game, he got a hair cut or something cause his hair just isn't as bushy as it has been in every game and looks sorta dumb.
X has armor parts, as always, but its done differently in this game. He only has 2 looks, the Neutral Armor look and unarmored look. There's two "sets" of armor in this game. Don't be misled by my use of the word "set". With the Neutral Armor you can use any combination of parts from both sets and from the standard equipment, meaning you don't have to collect an entire set to use it as you did in X5 and X6. When you select all parts from a set, the Neutral Armor will light up, red for Hermes and blue for Icarus. Also, each part of the Neutral Armor will change color depending on what part is selected. The Ultimate Armor, unlocked once you beat the game with 100% on X, looks like the Neutral Armor with a purple and blue glow, you cannot select any parts when you select this. The Ultimate Armor functions a little differently than it did in X4-6. It cannot hover, it has unlimited uses of Nova Strike but it has a cool down between uses, it makes damage you recieve minimal and no matter how much damage your receive, recovery health will never go below full unless you switch him out and back in without letting him recover (this is not the case in the ride chaser stages while actually on the ride chasers), you run fast and jump high, essentially giving you both Icarus feet and Hermes feet parts and you have a long ground dash.
In most Megaman X games you would collect Heart Tanks and Sub Tanks in the levels, and later ones you would collect Chip Upgrades and Weapon Energy Tanks as well as a Weapon Tank and an EX Tank. And in X7 you got most of these through rescuing Reploids found in dangerous spots in the levels. In X8, not only has Reploid rescuing been eliminated (thankfully) but so has searching for subtanks, weapon tanks, etc. The EX tank has been eliminated altogether and you have Retry Chips instead. In normal mode you get two free Retry Chips that replenish every time you start a stage and in Hard Mode you get none. Not to worry though, you can purchase 3 Retry Chips from the R&D lab on any difficulty with metals obtained in the stages. Metals are also used to purchase all upgrades, such as Zero's weapons, Life Ups, Subtanks, Double Barrier, etc. You get metals from pretty much anything you defeat. There are also metals of higher value strewn about every stage and you get 2500 metals for any boss you defeat. There are also mini-games called "Intermissions" on 4 of the stages that allow you to get massive metals. All of those efforts are pretty futile though when you can just use the glitch to get 10 million metals -1. Simply purchase a refill for the subtank when you don't have enough metals.
There are two new navigators in this game, Layer, who specializes in boss data, and Pallette, who specializes in secret routes, and of course Alia returns as being the halfway point between Layer and Pallete. You select which one you want to use at the beginning of every stage when you choose your hunters.
As far as additional secrets, in your second game, you can purchase a weapon for the navigator you used most in your first game to make them playable, and weapons for the others after you use them to defeat 4 mavericks with. Beware though, if you choose a navigator as one of your hunters you will not see any dialogue, this includes story dialogue as well as navigation dialogue. Also if you have Megaman X Command Mission data on the same memory card, you will fight Cutman on normal mode and hard mode right before Optic Sunflower at the Troia Base stage. Fear not those of you who bought MMXCM for gamecube instead of PS2, there is a code that lets you fight him anyways.
In this game, the Double Team Attack has been introduced. You have a meter that increases as you deal damage (and maybe take it too) there is also an item sometimes dropped by robots you destroy that looks like a chaos emerald from sonic games and is red and blue that will increase the gauge by a large amount. Once it is full you must successfully hit R2 and hit the target with a small sphere that expands around you. If you succeed you will enter a matrix styled numbers on black back ground where both your characters stand on a circular platform and just wail away at the target doing massive damage. If you miss or hit the target's barrier, no worries your gauge will reset to 0% but will fill back up to 100% in about 5 seconds. In all honesty, this is about the only feasible way to get the final hit on the last boss with Axl is to have him lead a Double Team Attack after the barrier is down.
In this game the bosses all have a Hyper Mode attack which you cannot defeat them until they perform it and they are invulnerable while performing it, which adds some new challenge to the game. Old fashioned ride armors return in Bamboo Pandamoneum's level, equipped with a paralyzing laser and punches, as well as a charge up punch combo. They also reflect most enemy attacks and destroy just about anything they land on. The story is well done and progresses a little after every stage you beat. You can skip the story segments and cutscenes if you wish. Each character has a segment in the ending that you only get to see when you beat the game with him. I read that Axl has a hard mode exclusive extra on his ending, but I did not see it. Hard mode does however bring some extra scenes with Vile and Sigma in the final stage.
The music in this game is great as always in the Megaman X series. It retains that post apocalyptic feel that we have come to expect in an X series game. The sound effects are also done nicely, nothing at all annoying and reptitive. The voice acting is wonderful, perfectly done. Infact, I think it was done so well that they decided to use most of the same voices for Maverick Hunter X. There's so much emotion displayed through the voices rather than how X7 was where it just seemed like someone was reading off a script with no direction at all.
In summary, this game kicks utmost ass and you need to buy it NOW whoever you are. I don't care if you hate Megaman. BUY. THIS. NOW.