D&D's classes are so iconic at this point that they hardly need introduction; classes such as fighter, paladin, cleric, and wizard are part of the lingua franca of fantasy rpgs at this point, defining the character archetypes expected to be available to players. Each class comes pre-packaged with its own fictional conventions. For example, if you're thinking of playing a strong warrior who relies on heavy armor and a shield for protection, you pick fighter or paladin. If you want a more agile combatant, you probably opt to make a rogue or monk instead.
Subclasses provide a nice bit of differentiation between characters of the same class and expand the available options. They're necessary, as 5e has been quite reluctant to introduce new base classes. (The only new class to appear outside of the Player's Handbook is the artificer introduced in the 5e Eberron book.) A good subclass can make a class that was previously unappealing seem interesting and worth a try. That said, not all subclasses are created equal in 5e. Without house rules, the Way of the Four Elements monk subclass is notoriously disappointing, and the pre-errata Beast Master ranger in the PHB doesn't really satisfy the fantasy of having a powerful animal pet.
Additionally, it's a shame that many of the subclasses are held off until you reach third level; this can lend the feeling of not really getting to play the character you want for the first few levels. On the other hand, the first two levels in 5e D&D go by fairly quickly, often in about three sessions, so it might not be too arduous of a wait. Nevertheless, it does feel strange that some characters get their subclasses earlier than others.
Expert paths function a bit like 5e's subclasses. You pick an expert path when your party reaches third level to further specialize and tailor your character's abilities. You might pick an expert path that builds on the abilities you already have, such as picking the fighter expert path to emphasize your warrior's martial prowess, or you might pick an expert path that gives you access to a totally different suite of abilities, such as picking the sorcerer path to add some magic to your warrior. Your expert path provides additional abilities at levels six and nine.
When your party reaches seventh level, you pick a master path for your character. Master paths are essentially "capstones," providing one final piece of differentiation for your character. As with expert paths, you have free rein to pick whatever master path appeals to you. You might top off your fire-wielding magician by picking the pyromancer master path or polishing off your archery-focused rogue by picking the sharpshooter master path. Your master path provides additional abilities at tenth level--the final level in SotDL's progression.
D&D has a potential issue in that there are no meaningful choices to make about your character's growth once you've picked your subclass; after that, your character's mechanical development is more or less set in stone. SotDL solves that issue by allowing players to shape their character over the course of their growth at steady intervals. Since there are no restrictions on which expert or master paths you can pick, you aren't bound by a sense of linear progression. There are hundreds of expert and master paths scattered throughout SotDL's supplements, so the amount of personalization and variation you can achieve is astounding.
Additionally, although SotDL adds a ton of options through its path system, it may still suffer from one issue that 5e also shares: you may have to wait for quite some time to feel like your character is really fulfilling the archetype you have in mind. It's entirely possible to make a character with a background in entertainment who focuses on song magic, but if you want to play a "bard" and will not be satisfied until it says "bard" on your character sheet, you're going to have to wait until seventh level because bard is a master path in SotDL. Similarly, if you want a "beastmaster" ranger, you're going to need to bide your time until the master path for that becomes available. That's an even longer wait than what D&D's subclass system makes you endure; seventh level is basically the start of SotDL's endgame tier--the beginning of the end, so to speak.
Of course, one thing that has to be addressed is the question of balance. It's here that an interesting difference in culture between fans of the two games is evident. Hardcore D&D fans on the internet seem to constantly moan about how some classes are "overpowered" and others are so underpowered that they are "broken." I generally think these concerns are overblown (most casual D&D fans don't even notice any power disparity between the classes in my experience), but there are issues that can come up in play. The paladin class, for example, does just seem to be better designed to fulfill its role than the ranger class does, for example. That doesn't make rangers unplayable, but it does feel like an area of 5e that didn't come out of the oven fully baked.
Also, it's odd that magician is the only novice path that doesn't ever get a boon to use for its core shtick: casting spells. This means that rogues and priests can potentially feel like more proficient spellcasters than the class that supposedly focuses on that ability. Again, the rogue rears its "pick me!" head: the rogue's ability to get a boon to attacks can apply to spell attacks and their bonus to damage can apply to spells--the latter another thing that neither the magician or priest get. Although the magician may get access to more magic than the rogue, the rogue's combo of bonus to spell attack rolls and spell attack damage sometimes makes them feel like the superior spellcaster in a lot of situations.
Balance issues are potentially exacerbated by expert and master paths that can vary wildly in power level. Remember, there are literally hundreds of them; it beggars belief to think that they have all been exactingly balanced against each other. It's hard to really say that paths that grant abilities to instantly slay a foe if the conditions are right, such as assassin, or the straight-up buffs that fighters get are equivalent to the bonus to nature skills and the ability to teleport between trees that the druid gets.
Again, this doesn't mean that some paths are "traps" or that others are godlike in power, but it does mean that not every path is the equal of its peers. The disparity only becomes more apparent when you look at the expert and master paths from the less well-vetted supplements that followed after the core book, some of which have absolutely mind-bogglingly more powerful abilities when compared to the initial paths in the core rulebook. This also happens with some of the variant novice paths that have been written for the game; as an example, whenever the spellguard path comes up in conversations on the SotDL Discord...well, get ready for a lot of bitching about it. To be honest, "power creep" is an issue.
The insistence that SotDL's paths are finely tuned things of beauty contrasts badly against the amount of conversation in SotDL spaces about "builds" and optimization. Ironically, this is the sort of thing that SotDL fans often accuse D&D fans of being obsessed with, but it's not at all uncommon among SotDL fans in my experience. Here's an example from the SotDL reddit:
Power gamers and minmaxers, like the poor, will always be among us. I think Jesus said that.
Instead of addressing that issue, SotDL merely reverses its polarity. It's noticeable that SotDL's "martial paths" tend to deal more damage with their weapon attacks than its spellcasters do with their magic. Low-level damage-dealing cantrips feel like a poor choice, since they are a limited resource (no all-day cantrips here) that deliver damage on par, if not inferior to, basic weapon attacks. Even taking into account access to "area of effect" spells (which have their benefits mitigated by the fact that's better to deal a killing blow to one foe than it is to damage multiple foes due to the vagaries of the action economy), spells just aren't on par in a game that is, like the D&D chassis it's built upon, focused on combat. The end result in SotDL feels like an over-correction to D&D's fetish for magic This issue is only more pronounced when the characters face higher difficulty monsters, which tend to halve damage taken from spells.
One thing I do tend to like about SotDL's paths is how condensed they are in terms of information and the delivery of mechanics. As an example, consider the D&D paladin's Lay on Hands ability (on the left below) versus the SotDL paladin's Faith Healing ability (on the right below). They have the same intent, but the SotDL version is much terser and less "fatty" in terms of verbiage:
