I reckon its interesting how varied the responses are here. Different people dislike different parts of the game. Another way of looking at it is that different people *like* different aspects of the game. It's a great game with broad appeal, and not everyone likes the same things.
I've read through a bunch of posts here. There are some where I could kind of agree; but many of the things people are saying that don't like are think I reckon the game *needs*, for variety, pacing, sense of progress, challenge etc. I don't really want to go into specifics, because I don't want to look like I'm sniping people's opinions; but I do think its worth pointing out that having 'bad' things that the player can end after making progress can make the game feel good to play.
[end of rant]
Now my answer to the question: I reckon swimming in lava looks and feels a bit crap. It's at the climax of 'pre-hardmode'. An important milestone for the progress of the game... and it just looks unfinished or something. The player is basically invisible behind the lava, but when you push into the side of a wall or something part of the sprite is shown peaking through. It looks bad. Also, there's no suffocation for some reason. I suppose that's just to be nice to the player, so that they don't have to teleport out in a panic when they can't work out where they are... but it just feels a bit weird.
I reckon lava swimming would be better if there was a visible silhouette while submerged, and with the same oxygen rules and honey and water.