I like that, honestly. It feels like we're going back to gen 4 or 5 in terms of battle dynamics.
The major impact of Dynamax, I feel, will be to make reactive play more viable and thus make proactive play less important.
A major problem with OU play has been matchup dependence: too many of the important decisions are made before the battle starts, to the extent that a lot of matches are either decided at team preview or decided when one player incorrectly predicts which of a million possible sets a certain threat is using. By the time you actually know what the opponent's strategy is, it's often too late to stop it. Z-moves made this a lot worse by giving top-tier threats a whole array of ways to muscle past things that would normally be good answers. With Dynamax, decisions which are currently made during teambuilding (which Pokemon you're gonna use as your ace, how you're going to deal with specific threats) can be more feasibly made during play.
The lack of an item restriction is important for this, because items (and consequently megas and Z-moves) have to be chosen before play whereas your choice of Dynamax doesn't. Dynamax being at a lower power level is partly a result of this -- you can't have the mechanic be as powerful as megas or Z-moves if there isn't as big a cost associated with using it. But Dynamax's power level also benefits a more reactive play style, because it doesn't usually have enough raw power to be worth building your whole team around. Conversely, if you do decide to build around it, your opponent won't be as locked in to their strategy at team preview, and will thus have more options to counter whatever it is you're doing. (It's probably not a coincidence that every Dynamax move is cancelled out by another Dynamax move.)
I see three things that are made less potent by Dynamax:
1) Powerful setup sweepers. The defensive buff from Dynamax looks to generally be stronger than the offensive buff and you can't use status moves other than whatever Max Guard does, so a battle between two Dynamaxed Pokemon will be slower than current battles. That and the HP buffer generally make it harder for a setup sweeper to just OHKO your whole team, and certain Dynamax moves can directly reduce your opponent's stats or buff your defences to stop their momentum cold too.
Sure, there will be some Max moves which give offensive buffs, but they'll take time to accumulate and will be limited to certain types (and potentially stopped by immunities). I for one will be glad to see less games where a Pokemon gets one free turn, uses Extreme Geomancy Bellydance and then instantly TPKs the opponent.
2) Weather/terrain abilities. With so many Dynamax mons being able to change weather or terrain on the fly, that both gives you more options for setting the field effect and more options for your opponent to get rid of it. Weather and terrain abilities have completely dominated every meta they exist in with relatively little counterplay unless you've really prepared for it or are using a weather/terrain of your own, so this is a welcome change.
3) Constantly switching. Once you've Dynamaxed, you're committed, because switching out while Dynamaxed wastes your remaining turns of it. I'm undecided on whether this is a good thing or a bad thing.
I'll admit that the mechanical utility doesn't really match up with the aesthetic Dynamax has going on, but I guess that's easy for me to overlook because I much prefer the current implementation to the idea of a mechanic which is basically "you can use three Z-moves in a row and they're even more OP than before". Personally, my biggest annoyance with Dynamax is that some Max Moves are just clearly better than others, in a way that so far doesn't really seem designed to balance the types.
