My biggest gripe with Gen V is that it introduced nothing new mechanics-wise.
Gen I introduced IV's, EV's, TM and HM moves, and most other basic gameplay mechanics.
Gen II introduced Held items, breeding and egg moves, move tutors (In limited form), a special attack/defense split, a redone crit system to replace the speed-based one, weather effects and the first field trap- spikes -, 2 new types, and rebalanced the rest of the type matchups
Gen III introduced a new EV system, Natures, and Abilities, off the top of my head.
Gen IV introduced a Special/Physical split for specific moves, rather than associating types with a particular attacking stat, and online battling, though the latter is not quite as important.
Gen V is just, more of the same. More abilities, more moves, stuff that is ho-hum and relatively minor. There is still the random nature, problem easily identifying, and overall imbalancing nature of the outdated IV system (as well as its association with Hidden Power as it stands) and the "expansive roster problem" that comes from the methodology of Gamefreak's balancing process. Rather than simply giving characters who do not preform as well as others numerical buffs and specifically unique niches for the game, as most developers do when balancing a game, Gamefreak tends to homogenize the approach by adding more and catering gameplay around what was added, rather than melding it with the old. Oh, Dunsparces stats just aren't up to snuff for it to fill its evident role? Let's make a new pokemon that does what Dunsparce did, only better, so that role can be filled. [Pokemon Name] Is dominating the metagame? Well we have a solution for you! It's [New Pokemon Name]! The new Pokemon that is even better than [Pokemon Name]! Now [Pokemon Name] has some competition for its role! Luckily [New Pokemon Name] doesn't learn two of the moves [Pokemon Name] can, so it gets that niche filled, at least until next gen when all the other Pokemon get to learn those two moves and [Third Pokemon Name] comes along to outclass both of the other two.
One does not even have to look at what could be considered a problem with the game for ideas of "what to change." Most games evolve and experiment as the series goes on. Even if a new mechanic isn't well received, it's a part of the process that goes into designing a long series, and it is something that needs to be done to keep games fresh.
I love the Pokemon series, which is why I am so saddened to see it treading in a rut and afraid to take risks. I am not a fan of Stealth Rocks, as a small example, but I can see where the developers were trying to make something new and unique. The problems were obviously seen, and there was an attempt to solve the problem through the introduction of more fighting and steel types than any other generation, as well as the changes made to Magic Coat and the new ability, Magic Mirror. The problem with all these changes is that in attempting to build around the metagame of the previous generation, the game seems to forget that it can offer something new as well. It is that staleness that makes continual generations look less and less appealing, as less and less is being added or changed about how the game goes about its conventions. We've already seen what the last generation can do, now let us see what new things the next one can do.