Important stuff that is different from 4e and then a short review at the bottom:
- Icons and One Unique Thing take the place of Themes/Paragon paths, defining your place in the world/campaign/protagonism and provide players with some narrative authority (Background Traits in 5e have some similarity here as well).
- Theater of the Mind style combat with resources, control effects, and abstracted movement centered around that paradigm. Due to the latter (and this is probably the major area of divergence between the two systems), forced movement (or at least with respect to the intensity and scope that it manifests in 4e) is basically gone.
- An Escalation Die is a focus of tactical play and ramps up the lethality of PCs and opens up their options as rounds accrue.
- The "encounter power" paradigm has been replaced with a recharge paradigm.
- Utility "powers" are much more scant or their payload is disseminated differently; eg - one Utility ability, such as Animal Companion, takes up most of the heft of your class's inherent scope of Utility.
- "Second Wind" (Rallying) can take place more than once per fight, pending the outcome of a check. Outside of that, the Healing Surge system is pretty much ported over. However, beyond Rallying, given the dearth (or lackthereof in some cases) of Utility "powers" present in some martial classes, the intra-combat opening up of their surges (Recovery) will be the primary domain of "Leader" role abilities. That being said, there are some encounter/recharge options to add self-Recoveries (surges) as riders to a few classes' attacks.
- Rituals are no longer codified but are improvised, flavor/mechanical negotiated versions of spells you know. If you don't know spells, Ritual Casting is unavailable to you.
- Races are pretty much ported.
- Considerably less EoYNT.
- Considerably less "immediate actions".
- Less Conditions and the ones that are similarly named are different in a way, or completely so, and are generally less "fiddly".
- Lack of unified class mechanics and resource scheduling (eg momentum for the rogue).
- Level bonus to checks/rolls vs 1/2 level.
- 10 lvls instead of 30.
- Rather than selecting trained broad skills, a Background pool of points is spent on careers/exposure etc which create applicable "skill bases" to be leveraged in play.
- No Reflex Defense.
- Middle score of 3 for several passive modifiers rather than the highest of 2.
- No Warlords.
- No Dragonborn. Errr. Yes, there are - see optional races - Dragonspawn.
- No Condition/Disease Track.
- No Swarms so no Swarm mechanics to use as a tool for various things.
- No Wealth/Level.
- No complex conflict resolution framework (eg no Skill Challenge). HOWEVER...there are objective DCs for the math of the system that are expected to be used so porting over the Skill Challenge framework would be trivial.
I think that is most of the important stuff. If you used Themes/Paragon Paths/Epic Destinies to thematically guide play in your 4e game and allowed your players narrative authority/assumptions that they could invoke/rely on/use as insurance, then OUT and Icons may not have an enormous difference on the functional play at the table. If they were just PC build tools to facilitate combat, then this will be new. Given that they do have acutely specified thematic focus (and Icon interaction has some mechanical heft), there will be some distinctions twixt the two table experiences. The Escalation Die as a universal mechanic will quicken fights and bring some quality dynamism into play with its interactions with various class mechanics.
From a combat perspective, the engine is more swingy than 4e, with less precision, granularity, and dynamism with respect to tactical movement (and forced movement as it basically doesn't exist). However, the Escalation Die and its interactions with class mechanics will bring about some fun play and leveraging of trigger-based abilities. The movement system is quite solid for TotM and has some functional tactical overhead embedded in it. While tactically rich and effective, multiple SWs (contingent upon checks) doesn't make up for the lack of action economy leveraging HP restoration from Utility Powers, and other mitigatory deployments, I don't think. Overall battlefield control will be notched down by a fair stretch as well and, although there are good tools for Defender "stickiness" (and Intercept allows some general Defending), the rich, tactical depth of tanking and dominating battlefields indirectly will be stepped back. Monsters are very solid and roles are there but overall, they are not as rich as MM3/MV monsters and the fights they engender; but much of that is attributable to lack of forced movement. Math is tight. Encounter budgeting is good. Guidance on encounter design is good.
Again, while Skill Challenges aren't actually in the system, the objective DCs/lvl are present so its just a matter of porting the framework. Easily enough done. In non-combat conflict resolution, players have a lot of "outs" or narrative authority due to the structure of the Background/skill system, OUT and possibly leveraging an ICON. Good stuff. Lack of Rituals for non-casters (save for one specific Ranger build) is troubling. Obviously, free-form Ritual Casting will have a high potential to perturb the "spotlight propensity" toward casters in non-combat scenarios. That is theorycraft at this point but I'm sure I'm not the only person who has that concern. I should be able to port the Condition/Disease Track over and sort out how Swarm mechanics should work with this engine.
Overall, this is a great system and very lean and functional at what it is trying to achieve; a slightly swingier, less granular/fiddly (less robust tactically), slightly more narr-heavy 4e. Tons of the material is portable to and from D&D systems (specifically 4e). The expansion will bring more classes/races/feats/maneuvers/spells/powers etc but, just like 4e, the math is solid and intuitive and I could easily create well-balanced classes (etc) that are thematically and mechanically robust, immediately. The best thing, I think, you can say about a TTRPG is that it knows what it is trying to do, it expresses plainly what it is trying to do and it then proceeds to do so. 13 Age hits on all 3. Great game system and I'd expect nothing less from these creators.