Most of the OP has been answered already, so I'll just make one point. I think that it's kind of foolhardy to judge the potential variety in Pokemon by the variety of viable Pokemon. Instead, we should judge by the variety of viable teams. In the current iteration of the
Victory Road viability ranking thread, there are 19 Pokemon in S and A+ Rank combined (counting Deoxys as one Pokemon). 19 choose 6 = 12996271411200 possible teams. Most of them probably suck, but hopefully you get my point here. And that's just from S and A+; the lowest rank commonly considered tournament-viable is B-.
(Also I haven't used Rotom-W in XY OU. Just sayin'...)
I did not play a lot of games on XY, perhaps about 350, with most of those games in the early stages, with about ~ 60 on late May and early June for two weekends as I did not battle since late January. So I did not use many Pokemon either.
I remember seeing Deo-D, Bisharp, Aegislash, Lando-I, Mawile, and Thundurus-I on a few teams. Of course, the usage of Pokemon can be correlated or anticorrelated with each other. Regarding the latter, the many competitive players do not want a team that has 4 or more Ground weakness (especially since Earthquake is a really good physical attack this Gen), and many Pokemon have synergy due to their offensive traits, defensive typing, and abilities. Thus the real number of viable teams is really quite constrained, and there are numerous reoccurring strategic motifs (known as "cores" in competitive Pokemon and in chess this can be seen as the development of pieces during the opening stages) that a competitive player should be expected to play around with his or her respective Pokemon.
The competitive aspect of the game, one that I found the most enjoyable, much more than the battling itself, was considering all these "ifs" in battle and formulating possible responses to these various scenarios and analyzing if my team is equipped to deal with these contingencies. I do enjoy the process of optimizing teams and preparing them for the expected challenges of the current metagame. During the battle, one is expected to evaluate one's position in the battle and respond appropriate with his or her resources and one should be aware of the inherent information asymmetry in the EVs spreads, items, and movesets of the Pokemon involved in the battle so you can exploit it if your Pokemon has a coverage move that can lure out a problematic Pokemon on your opponent's team, or that you would not expose a valuable Pokemon to another that can potentially KO that Pokemon.
Competitive Pokemon is much more intellectually demanding (and competitive) than most people's initial impressions of it. I do not want to play it, not because it is competitive, but there were a few months were I just ruminated over the possibilities of many battle scenarios, and I was unproductively engrossing.