I'll try to keep this spoiler-free.
- You build an army, and you get a HQ to house them.
- You get to name your main character, the army and finally the HQ. None of them are voiced in dialogues, as usual. Others can also refer you with pronouns in voice, despite your name being printed in the dialogue texts.
- Voice acting is in. Both in cutscenes and dialogues.
- Fight, defend, items and unite attacks are back intact.
- Armours, accessories and items are also intact, but your inventory is unlimited like in IV. There is no warehouse feature in the HQ.
- Characters regain all HP and MP at level up, like Tactics.
- Formation is most similar to Suikoden and II, with 6 available spots and fully functioning S-M-L weapon ranges. The formation system from V is gone. However, you can only have a maximum 4 fighters and 1 support at any one time.
- EXP gains differ according to character levels, as always.
- Similar to V, your HQ levels up with the plot, not with the amount of Stars recruited.
- Movement on the world map is similar to Suikoden III and Tactics. There is also a calendar system with months and days, but the impact on the plot is minimal. Presumably it influences trading prices.
- Town exploration is via a menu like Tactics, but you have the ability to walk through each shop/inn/house/town square/etc, instead of shopping/interaction entirely from menus.
- The classic three modes of battles are absent, and instead it follows Tactics' approach. Duels are fought one-on-one using the normal battle system. Army battles are simulated using multi-party fights similar to Luca Blight and the siege of Fort El-Eal.
- The Quest system is similar to Tactics, including Dispatch ones. The success criteria for them are still as vague as ever.
- The Slash Rune/Flash command is back from IV, but instead of a rune, a Support character makes this extra command available for you to quickly get rid of weaker enemies. That said, I haven't discovered the equivalent to a Champion Rune.
- No more Bribe option, but there's a Support character that makes the Escape command 100% effective at a price.
- Entourage system is gone, but nearly all must-have tag-along characters do not require a dedicated party slot.
- Enemies do not drop potch, only trading items. You mostly gain more potch either through quest completion or selling those at trading posts.
- Key cutscenes are replaced with animated sequences. Even better, for the first time, the HQ comes with a feature to replay cutscenes.
- There are no antiques in the game, and with them the appraisal feature is gone.
- Teleporting is available, but the available locations are very limited.
- The Mark of the Stars replaces the runes in the game, which unfortunately include the true runes. Each character comes with their own specific list of marks that they can equip in the available four slots. You cannot buy, sell or trade these marks as they are not items, and new ones become available exclusively through the plot.
- The weapon sharpening system is gone, and now each character can choose from a variety of weapons (including bare), but the types available are tailored to individual characters. Weapons now also have types (Slash/Strike/Pierce like) with matching enemy weaknesses.
- HQ features removed altogether: bathhouse, newspapers, sound settings, mini-games
- The HQ library is now replaced with a glossary describing all the different important NPCs, factions, countries, terminology, etc.
If I missed anything or you'd like to know anything specific, just ask here.