I had the opportunity to play Legends of Runeterra today, I only got through the tutorial and a handful of games with a deck I built myself. I didn't play very much, but I am going to share my opinion regardless.
Pros
The gameplay was pretty fun, it was a nice mix of Hearthstone and Magic: the Gathering. The game moved pretty fast, and there wasn't a whole lot to keep track of on the board. But it was also very strategic and complex with the "instant speed" spells and the way attacking/blocking worked. I think it will be very popular amongst people who thinks that there isn't enough strategy in Hearthstone, but also aren't invested enough to play Magic:Arena.
The game also seems very F2P friendly. When I first started I only had 20 cards from each region, so I couldn't really do much to build a deck. But I got rewards pretty fast. After just about every single game I played I got some cards. I am sure the rewards will slow down after I have played for some time, but Riot games seem to be very friendly to F2P players from what I have heard, and I don't imagine that Legends of Runeterra will be any exception.
One thing about the game that struck me as odd, and I am not quite sure yet if this is a pro or a con, is that you don't have an avatar or anything to represent yourself. All you get to see of your opponent is their username, and the champions they are using in the beginning of combat. I kind of like it because different skins really just feels like a way for the company to make some money by feeding into all of our inner collectors, and social features such as emotes or a chat system really just feels like a super convenient way to BM your opponent.
Cons
One big thing that I really don't like about the game is that there is almost zero indication on a card as to which region it belongs to. There is a tiny symbol in the center of the bottom of the card, but that is it. No colored border or anything this both makes it hard to associate a cad to a region, but it also makes it hard to associate a region with the cards. Regions already have their own colors, so why not try and decorate the cards with their region, so that at first glance players can know exactly what region a card belongs to.
Another thing that bugs me is how the cards are worded, it is sometimes difficult to figure out what a card does. I don't like how the card texts are in first person, I guess they were trying to make the cards feel more alive or something? But it makes it weird to read. I would prefer something more professional sounding, so instead of saying "Level Up: I have damaged the enemy nexus" it would say "When this unit damages the enemy nexus Level Up".
Another thing that could use some small improvements is the UI, not just in the home menu and when flipping about your collection, but also when playing the game. One thing that Hearthstone does better than any other digital card game out there is their super friendly, easy to use, animated, colorful, and lively user interface. This is one of the biggest reasons, if not the biggest reason, the game is so popular. It is simple and easy to navigate, you can go to your collection from just about anywhere, and all of the game modes are in the very center of the screen. Legends of Runeterra's UI isn't nearly as easy, to start a game you first have to select a deck, and only then can you see the "start a match" screen, and you can change the mode you are playing on with tiny buttons there.
Also during the game everything feels a little cramped. Your hand, mana, and board were kind of smushed into one end of the screen to make room for the HUGE combat area. The mana being off to the side is okay, but I do think it would make things a little easier to keep track of if they got rid of the combat board and gave more room for your units on the field. You could see which creatures are attack with a large orange arrow or something along those lines like there is in MTG:A, or perhaps you could simple 'fade' the none-attacking creatures, or you could do something to make the attacking creatures visible without having to devote so much space to it.
Summary
I think it is a very fun game with a lot of potential. The gameplay is great and they hit just the right middle point in terms of complexity. But there are a couple of "quality of life" things that could make the game so much more appealing. Things like a easier way to identify the region of a card, or a smoother UI, it is things like this that can change a boring game into an addictive one. I hope that the game receives updates to the interface and graphics so that it can play a lot smoother and be more attractive overall.