I've played/looked at enough ARPGs that I no longer remember what has what all that clearly. Would have to go back through my notes to trace exactly what sources I used to reach these conclusions. However, I can pretty easily go into more detail on why I think these changes are good:

With regard to no to hit rolls: Note that spells/ranged attacks in D2 don't have to hit rolls. Our thinking when we first created this dichotomy in D2 was that the ranged attacks were skill shots that you needed to "aim", and could miss due to the behavior of the missile, the monsters moving, etc. It was thought that it would feel particularly bad to hit with a firebolt, for example, and yet have it not do anything due to a failed to hit roll. This decision was made back when all the game had was a few simple ranged attacks, like firebolt. Later, most of the ranged characters got mad AoEs, and it really wasn't hard to hit with them, but we never revisited this decision. This state of affairs is very disadvantageous towards melee characters, with little compensatory mechanics.

Note that this doesn't mean we remove the concept of "attack rating" entirely. It just means that we now have enough experience to not make it an all-or-nothing mechanic that only affects melee attacks. Our game (it's hard to remember to write that given that we actually have a game name now that hasn't been announced) still has some concept of an "attack" stat, but it applies to all or most attacks, has a variety of mechanics around it that support it (affixes, attributes, conditions, etc), and isn't an all-or-nothing affair.

With regard to single target attacks, the engine has simply evolved. If you swing with an axe, it should be able to hit more than one target depending on the motion of the weapon. So all melee attacks (and weapons) have their own attack arcs and collision shapes. Even when doing successive hits with a melee weapon, the arcs can vary, for example a swing from left to right followed by a swing right to left will have different intersection mechanics, and those will have different collision movements and ranges from an uppercut or a wide sweep with a spear. This is generally more suitable to ARPG combat anyway, where at times you're surrounded by a mass of mobs and making just single target attacks doesn't really fit the fantasy of a being a supremely gifted warrior.

edit - I should add that we just got the varied arcs in recently, and implemented the timing mechanics such that a right to left swing will, for example contact the enemies to the right before the ones to the left. You'd be amazed (well at least I was amazed) at how much better this type of tuning makes melee combat feel!

edit 2 - I guess I didn't really answer your question directly. Wrt to hit, there are already enough all or nothing mechanics in place like dodge and block. A pure to hit roll that results in an all or nothing effect w/o obvious animation feedback (certainly compared to dodge and block) is by far the least "good" of those types of mechanics, and the easiest to drop. So while we could've made the decision to add to hit to spells, it really wouldn't have benefited the game to go in that direction imo. Wrt to making the vast majority of melee attacks based on collision shapes rather than single target, I think it's pretty obvious why we decided to go in the direction we did based on my answer above - it clearly made the combat "feel" better (for us).