These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Character Tiers: In a case which can be likened to that of Marvel vs. Capcom 2, the top tiers consist almost exclusively of Capcom characters, with SNK characters such as Hibiki, Geese, and Rock edging their way into upper/high-mid and the middle and lower/low-mid tiers mostly featuring SNK characters (cue jokes that, since Capcom developed the game, they intentionally gimped SNK's side). Unlike MvC2, the high and mid tiers are still quite good in the right hands, many of them even seeing their fair share of Tournament Play. The viability of several characters is actually independent of picks/counterpicks. On top of that, the Groove System adds another layer of depth (the strategies and nuisances of characters are bound to change due to the wide range of gameplay mechanics offered between each Groove), and while certain Grooves are better than others, "weaker" Grooves such as P-Groove and S-Groove can still be deadly up against users of the other four Grooves if utilized properly. It's not quite as unbalanced as some would have you believe.
Game Breaker: Roll cancelingnote quickly canceling the first three frames of any ground roll into a special or super; on top of this, the inputted move gains the properties of the roll, meaning that the RC'd move is invincible (although it can be thrown out of if the move isn't airborne or a move that already has invincible properties such as a DP). In fact, during the 2002 EVO tourney (in which 20 Japanese players attended), most of the Americans didn't learn to employ roll canceling; most of the Japanese players did, leading to Japan dominating the standings (the only top U.S. finishers were 3rd and a two-man tie for 7th), with Tokido blowing up not only the other Japanese finalist (Nuki), but America's top placer (Ken, who did know how to roll cancel). What mitigates RCing's claim as a Game Breaker (even if slightly) is that, aside of the practice necessary to perfect its execution, the skill is available to every character, some of whom can benefit from its effects when faced with a bad match-up. Roll canceling has widely been accepted as a legitimate factor in high-level play since. In fact, their removal in the GameCube and Xbox versions (EO) is one of the reasons those ports weren't received as well as the PS2 and Dreamcast ones.
It goes without saying that the "boss-tier" characters (Evil Ryu, Orochi Iori, Shin Akuma, and God Rugal) are banned from tournaments.
Oddly enough those characters aren't nearly as viable as the top tiers due to their incredibly low health (they die in 1-2 hits or a super), and the sheer number of options and moves in the game. Roll canceling, custom combos, etc, make it so many of their tactics can be easily nullified.