I've noticed this has been talked about a lot lately as well and stems a large part from Space Marines being a popular army in 5th edition currently. We talked about this previously and whilst they may look the same or feel similar to play against in terms of statlines and weapon loadouts, they are very different armies. That being said, a lot of the criticism levelled at competitive 40k gaming is the lack of variety in army lists seen on the table-top. This is probably quite heavily correlated with the massive amount of Marine armies and the belief they are all the same or at least very similar in playstyle but ultimately there is a lot of variety within competitive play - it may seem like there's not though.
Let's look at this. Look at the Army List section here on 3++. Even without focusing on all armies such as Imperial Guard there are a lot of lists there. Look at the old Best Of section on YTTH. More army lists and whilst there might be some parallels between lists, that's a very large pool of generally competitively oriented lists. Some armies do this better than others like Dark Eldar, Blood Angels or Grey Knights and whilst certain units may be predominate (i.e. Psyfledreads, Ravagers, RBacks, etc.), most of the 5th edition books have more than one competitive choice in each force organisation slot.
So why then do we hear people complaining about competitive play being stagnant? Perhaps people cannot differentiate between Marine lists and the differences within each list. A Razorwolves list with Thunderwolves is very different from one without for example but they may look very similar. More likely we can place this blame on the Internet of course - it's what everyone else does so it does sound like a good idea. What the Internet has done is allow for quick and easy circulation of so-called 'netlists.' The ironic thing is, the majority of people who take lists directly from the Internet don't play them for long or if they get inspiration from such a list, they modify to fit their style. Guess which player type does better here? Yup the second one.
Whilst the netlists (or variations of) may be common, they also cycle through quite quickly. How regularly do you see Imperial Guard or Vulkan Marines nowdays? Still good lists and players who actually enjoy the list/made it their own, are still using them and often doing well with them (it's like experience correlates with success or something). Rewind a couple years though and there they were in their hundreds all over the world. Now? Lots of Grey Knights with Purifiers and Razorbacks and Psyfledreads. Ensue Internet whining! This is the cycle of netlists though - people with expendable cash or ability to easily switch armies or people who just want the newest and greatest will switch over to what is populated as the 'next best thing' which is generally a net list. Lash Chaos? Nob Bikers? Fatecrusher? Seer Councils? All net-lists and how often do you see them these days? Regardless of the competitiveness of the lists (or lack there-of) the extreme lack of these lists (I would hazard maybe one or two of these lists would show up at a 30-50 man tournament) compared to how common they used to be is evidence against stagnation. The competitive scene has evolved and thus removed such lists from contention. Yes they have been replaced with other net lists and in time they will be replaced as well (though again, the players who have made them their own or only used the 'netlist' for inspiration will generally keep playing them) - ergo evolution and not stagnation.
Yes there are a lot of Marine armies out there with a lot of players and it can get boring as hell playing MEQ after MEQ after MEQ - even if they are hugely different. In terms of hugely viable 5th edition armies with as many bells and whistles you could want you've got a bunch of MEQ armies, Imperial Guard and Dark Eldar. Some lists like Dark Angels and Tau can put together respectable lists and others like Orks and Tyranids aren't OMGWTFYOUSUCK but yes, Marines are common and whilst they may appear the same - they are not. Hopefully Games Workshop's release schedule will fix this over time and significantly divergent lists such as Eldar, Chaos, Daemons, Tau, etc. will all come back into play. Until then, well sometimes you're going to have tournaments where you play a lot of the same army or similar army type. Look at my Event Horizon games - four of the six were Imperial Guard based and whilst every single one of them had Chimeras and at least one Leman Russ, they were significantly different (balanced IG mech, Ld bomb, Chimera + Russ spam, IG Hybrid + SoB). Yes it was a bit repetitive to play against the similar army each time and sometimes this happens but the lists were at least different and this has only happened to me in one tournament. Let's do a quick look at the top 8 lists at NOVA.
We have a Hybrid Space Wolves, Venom Spam Dark Eldar, Draigowing Grey Knights, Loganwing, Coteaz + Dread Grey Knights, Mecha Imperial Guard, a Grey Knight list I don't know (Devin's) though I think it was Mech based (may have had Crowe) and a Hellion/Reaver Dark Eldar list. That's the top eight at one of the most competitive tournaments in the world and none of them are remotely similar. The closest are the two Dark Eldar lists though Dash's list is clearly massing Venoms + Ravagers whilst Adam's has some token Venoms plus the usual compliment of Ravagers and then units not seen in Dash's list. There's obviously some variation here and whilst we're going to get some duplication as we go down the field (there are a finite number of decent army builds), a spread this diverse at the top of a very competitive tournament indicates competitive play isn't stagnate.
Again, you may get runs of similar or the same armies and with so many Marine armies out there, this feeling may be more prominent. In terms of list competitiveness and how many competitive armies each 5th edition codex has produced though? Many many more than ten per codex and whilst one or two of these builds may become extremely popular by the Internet (aka 'netlists'), the average lifespan of such armies is generally pretty low as their turnover rate is so high. Again, an indication of an evolving paradigm and not stagnant.
