The Belt of Giant Strength sets your strength to a specific value, like in AD&D. I think this is a bad idea, because it will make players feel like they wasted an important resource. Imagine giving up a ton of advantages to get your character up to 20 strength, and then finding a belt that boosts it to 21, and realizing that you could have gotten feats and better ability scores all around if only you'd known that you'd come upon that magic item. This item has already been discussed extensively earlier in this thread.
So analyzing where various parts of the game come from:
AD&D elements:
- Philosophy of magic items (rare, and nearly impossible to buy and sell)
- The Power of the DM: In 3.5, there were so many rules that the DM didn't have to make a lot of rules decisions. From what I've heard, 4E takes that to an even greater extreme, making D&D almost videogame-like in its precision. 5E goes back to earlier editions, where the DM has to make a lot of decisions. For example, the DM decides which ability score to use for a saving throw, or what kind of ability check to make and when it's appropriate. I think this will make the game go a lot more smoothly with a good DM, but may result in arguments with an inexperienced DM. I wouldn't be surprised if some precision is added before the final game is published.
D&D 3.5 elements:
- Ability scores. The same values and modifiers are used as in 3.5, though there's now a cap.
- Spell balance. The power of the various spells relative to the power of PCs has a similar balance to 3.5.
- Skill checks: While skills have been removed, equivalent checks are based on ability bonuses, as opposed to AD&D's "I have a proficiency and therefore I'm good at skill X."
Pathfinder-inspired:
- Per-level class abilities: Earlier playtests gave non-casters a very small list of class abilities at level 1, and that was it. The latest playtest has very few dead levels.
- Paths: Each class has multiple paths, much like Pathfinder's archetypes. This adds a nice bit of customization, and the paths have different enough abilities that, for example, playing an Enchanter is significantly different from playing an Evoker.
Hybrid 3.5/AD&D
- Feats: While feats have a lot of the same effects as 3.5 feats, they often act more like 2E's proficiencies.
New elements (or 4e, since I never played 4e):
- Spell saves and durations are no longer variable based on caster or spell level. This is a simplification of 3.5's math-heavy spell variables, and an improvement on 2e's tendency to give each spell its own special save modifier.
- Magic item flavor. Both the descriptive text and the powers of items are much more interesting than in previous editions.
- Lore: While skills are gone, knowledge skills still exist in the form of lore. This system is new to the latest playtest, and needs work.
- Backgrounds: Each character has a background, which is often a profession, which provides mostly role-playing bonuses, and gives the character a place in the world. Systems like this exist in other RPGs, but it's new to D&D.