So I was pretty far in Echoes: I beat it later last night. Played it on Hard Classic.
It is an interesting game and I'm glad my sister gifted it to me and that I got around to it now. I've never played OG FE2 but Echoes is in part a portal onto Fire Emblem before classical Fire Emblem was decided and a very different game. I imagine it also has its own idiosyncrasies introduced in the remake.
I'll start with the first difference that really presents itself: there are text adventure-esque town sequences where you can talk to NPCs and examine the premises for items and descriptive text. This is the primary way of both a) recruiting new soldiers and b) receiving items--there is a blacksmith that can upgrade gear but there aren't shops.
This bleeds into another striking distinction: there is but a single item slot for either an accessory, food, or a weapon. Weapons act as optional, albeit very useful, accessories rather than required tools. They have no durability. They can be upgraded. They impart weapon-specific combat skills. Among other accessories, you can even use shields, other than, say, the dracoshield common in later Fire Emblem. Food provides additional varieties of 'medicinal herb' and also plays into a fatigue system (more on that below).
Another glaring difference, similar to the first in a sense, is the existence of dungeons. Yes, dungeons in Fire Emblem. You wonder around labyrinths breaking pots and engaging symbols to face small battles. You can only take 10 people into the dungeon, unlike the normal battles where generally you can take everyone. Dungeons also operate on a suspend system--you can only suspend save outside of Shrines to Mila and the entrance. Combined with non-refreshing cogs (which give you the option to reset time a bit to counteract losses), a fatigue system (cured by food/offerings to Mila), permadeath, and some tricky enemies and dungeons can become pretty tense at their most potent, although that only really became apparent to me in the endgame.
Echoes is also, like Sacred Stones, a game with a map. It is interesting because unlike Sacred Stones, when your protagonists part ways, you can control them both, in turn, on the map. This allows you to swap between campaigns and there is also a token of interactivity between the campaigns. It is a neat feature: I found myself gravitating towards Celica's army more (heavy on swordsmen, Pegasus Knights, and mages) but it was really cool having two armies with their own identities. Another thing to note about the map is that certain bosses were really aggressive with their reinforcements, which would put pressure on you to try and finish certain parts of a campaign sooner rather than later. It also made it noticeable when they weren't aggressive, and there tended to be an underlying reason therefore, which was interesting world building. The reinforcements could get kind of obnoxious at times.
The classes and leveling are also quite different. There is a base villager class although most units start already promoted into a second tier class. Cavaliers, Mercenaries, and Soldiers have three tiers. The "Lords," Mages, Clerics, and Pegasus Knights have two tiers (DLC apparently adds a bunch of tiers). Upgrading classes brings you in line with a minimum specification rather than being a stock bonus. Moreover, lower level advanced classes are rewarded more experience than high level base classes. Particularly with the three-tier physical classes, this incentives early promotion. Class bonuses also have this affect, e.g. Dread Fighters getting a resistance boost and halved magic damage--quite important in this game--or Falcon Knights getting a plus 10 damage increase against demons.
I felt conflicted about raising my Pegasus Trio early and held off on them until they were about level 15 and I really needed that damage boost against the endless waves of demons that were being thrown at me. Catria and Est in particular seem to have quite good growths and unlike the other main physical classes, they weren't getting 40 levels or so anyway (if/when they reached 20 Falcon Knight). That regret lingered in the background tbh, but they were stellar units nonetheless. I had pretty much no qualms with the other physical classes when I set about doing it--many of those characters seemed to have relatively poor growth rates--or I just had bad luck--and they were still going to be getting lots of levels.
I got Alm/Celica and pretty much all of my mages to 20 before upgrading. Alm/Celica happened naturally really--I could wait on Alm after he could upgrade but it didn't take long to get to 20 and Celica, although her upgrade was mandatory, had naturally reached 20--and there was no way I was going to miss out on their insane growth rates particularly when they mostly had overshot their minimum specifications anyway. My best mages also had insane growth rates but, moreover, in Echoes you actually learn spells overtime (recall the changes to the equipment system) and you learn them faster as lower classes. Besides this, their advanced class abilities tended to not be as clutch as those of some other classes although Saint in particular had nice bonuses.
Some thoughts on the combat itself: Archers are quite different in Echoes: they have generally increased maximum range and can also attack at distance. Similarly, magic spells have more variable range. Mages/Archers can be quite dangerous and quite useful for this reason, particularly the former because it is a game of particularly low resistance. I like these ranged combat differences.
Demons are typically quite hard to kill with physical attacks, making magic (again), the Falcon Knight bonus, and blessed weapons very very valuable. There is also a growing focus on these conjurers that summon endless waves of demons, often in defensible positions, which you need to figure out how/when to blitz to stop the waves, which are also getting between you and your target. Nuibaba's level was particularly complicated in this regard by Medusa (spell), the cliff face, and her hangers on.
The turn-wheel feature is nice and speeds up a classic playthrough by limiting total resets. It is best in dungeons where it does not reset on battle's end.
Many levels are very small and bite-size, particularly in dungeons although those can also be tricky at times, particularly later on. It is a game designed around extra world map battles and dungeon encounters. Not entirely sure how I felt about this. It could inflate the time it takes to make progress and the steps involved relative to a normal SRPG in one sense while deflating it in another. I like a lot of the ideas the game explored, so I'm sympathetic in general, but sometimes I wanted a more traditional, meatier level. Which there are some in the game, just the long levels tend to be ones like Nuibaba's level, which, admittedly, I did enjoy in its own way.
I think the story is also interesting in how it departs from the Marth canon. Fire Emblems always do that to a greater or lesser extent but it felt particularly palpable here with Alm being a kind of proto-Ike in the sense that he brought class into play to an extent it usually isn't and then Celica's campaign very much had its own idiosyncratic goals and "feel." The game tied back to Marth canon at the end, imo but the way it got there was kind of different and strange, which I appreciated.
Edit:
Oh I left off two big things: spells/skills costing hp--this is particularly interesting for healers in the early game, with Healing and Nosferatu having a balance. Rings kind of make it obsolete. The other thing is I felt myself gunning for critical hits more often in this game--they seemed more likely more often.