Okay, as someone who played seasons 1-3, and then only played a few games to keep my rank in the last couple years on my two separate accounts, I'd really just like to make a few points after coming back to play more seriously again right at the end of last season. These points will be good and bad because I'm not quite jaded just yet and am still quite fresh after my break, almost to the point where I'd consider myself unbiased.
1) It's faster. Seriously, my match history has maybe two games that are longer than 30 minutes in-game time.
2) It's more carry-oriented. Back in season 1/2 the game revolved around the AD carry as the pivotal late game player, but after they decided to curb that role as being the most influential the game became very team oriented. This isn't the case any more as you seem to be able to carry from any role these days - even supports make a huge difference if they get fed in lane. Personally I don't like this as if one person gets fed these days they seem to be able to take on the entire enemy team and force an inhib by 15 minutes (I am currently using Amumu to carry my accounts back to plat with this method).
3) As mentioned in my last point, the inhib force. With the addition of herald, you can now take an inhib extremely early, which speeds up the game but makes it less interesting for me. If you can have super minions spawning by sub 20 minutes then the game is basically over because of the pressure unless you then throw on repeat. The reason I find this less interesting as I can use the same formula almost every game to lead my team to victory. That formula being - get jungle (arguably the most potent role at the moment), either get yourself or a trusted laner a few kills, watch them snowball, get early rift herald + dragon damage boost and choose a lane to push, push to inhib and take early using herald buff, get baron while supers are pushed and then ace the enemy team to end the game. At least in solo queue, it makes for an easy win.
4) Op champions. OP champions always have and always will be a thing. I was astounded to find that there are still only 3 bans on either team after coming back 2 seasons later though, as there are definitely MORE broken champs at any given time than in the past.
5) Most of the time, it takes longer to get into the game than the game takes. Seriously, the dynamic queue is fucking annoying. Nothing else to say here.
6) Itemization. They still can't get it right because they won't pre-define champions into specific roles. They always have and always will release items that are intended for one role and then are abused by another until the next patch because they're just plain broken (eg. spooky ghosts from this season, philosophers stone/heart of gold from days of yore). This fact is being compounded by the fact that there are a metric fuck tonne more active ability items now than from previous seasons.
7) The game is still fun, despite what I would consider flaws. However, I don't know if I will be able to play it competitively for a long period of time because overall the game feels less team-oriented these days and more 'hypercarry your teammates or someone else will'.