I'm guessing it has been mentioned in various posts here before, but I want to say something about it as well. The biggest annoyance for me in S/M is this: The lack of good training spots.
What is the main reason as for why HG/SS are my least favorite games in the series? Their lack of training spots.
On the other hand, why is Gen 5 my favorite and Gen 6 my second favorite (right now)? One big reason is the fact that they have EPIC training spots. With things like the Nimbasa Stadiums, Black Tower and White Treehollow, the Battle Chateau, the Restaurants (which I honestly never used much, but I know others have praised them), the PokeNav and Secret Bases, as well as several strong trainers that could be rebattled at various times, training Pokemon to level 100 (or any other level) was never difficult in these generations. In fact, it was always lots of fun, and I see it as something very positive if the actual level grinding in an RPG is fun. Plus a free Lucky Egg (or multiple), Pass/O-Powers, the changed Exp. system in Gen 5 and the buffed Exp. Share in Gen 6. Those things were amazing.
S/M took a big step backwards in this regard. There's daily battles with Morimoto, the Battle Buffet once a day, E4 rematches and wild Pokemon on Poni Island... then nothing more, at least I don't know of anything else. Which is a shame, as better training spots could have made the games quite a bit better. These are also the games that introduced Hyper Training, which requires training to level 100 before it can be used... Game Freak, sometimes I don't get you at all. I never even considered that the series would take such a huge step backwards in this aspect, which at least to me, is extremely important. Things like the Lucky Egg, the 6th Gen Exp. Share and the 5th Gen Exp. system (at least for lower levels) helps, but it could have been so much better.
Now don't get me wrong, I don't dislike S/M for this, I actually like them very much. But this does unfortunately drag them down quite a bit. Fortunately, it is pretty easy to fix, which I hope a sequel/third version/other follow-up will do... if there is one this generation.
I don't like to create negativity so I don't post in this thread very often... but now that it has expanded to all generations, there is one other post I want to make about various things I disliked in a previous game pair... but I'll wait with that for a while.