I think the grind/pay for classes is okay FOR NOW.
There's only two that you have to unlock out of a total of 8 characters right now and that's not so bad.
We'll see how it stands when we're much further in the game's lifespan like R6: Siege or League of Legends and it becomes necessary to have a wider pool of options to play as. Hopefully they don't fall into the traps those games made for themselves, locking new players out without paying or grinding an absurd amount.
The Dota 2 model is preferable when it comes to new heroes/classes (all characters free, only charge is for cosmetics), especially for latecomers to a game. I can't really commit to League or Heroes of the Storm, for example, over Dota 2 at this point because the barrier to entry with most heroes locked behind a pay or grind wall is too off-putting, even if I wanted to give those games a fair shake. A rotating pool of free classes isn't the answer either.
Why would I ever switch from Dota 2, a game I have over 1000 hours in, to try something else in the genre, if the gameplay experience is going to be negatively impacted simply because I'm new to the game X years in?
As for different game mode options -- I think a solo mode or something besides just teams of 3 will have to come eventually. The team of 3 dynamic makes sense and works well with how the game was designed for the moment (the respawning stuff in particular), but it would be nice to have some option that just lets you play alone or with different sized teams, even if the mode is very different and not considered the core focus. Launching the game polished with teams of 3 in mind is perfectly fine though, especially with how good the non-verbal communication is. The ping/callout system is maybe the best I've seen in any game, except for maybe Dota 2. Enough that you can quite reasonably play with friends or randoms without ever needing to use any voice or text chat, even if you don't speak the same native language. It's quite a feat, and I'm sure took a lot of time to nail, especially with how much is contextual.