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In 5e, you are limited entirely to the actions of the creature you have changed into, and cannot do anything beyond that. You basically become the creature's statblock.
In 3.5e, you can technically become higher levels so the actual effective limit of Shapechange would allow you to become a Wyrm Gold Dragon (CR 25) whereas 5e would limit you to an Adult Gold Dragon (CR 17). The 3.5e version is noticeably stronger- its breath attack does 22d10 (as opposed to 12d10), it has a fly speed of 220 feet (vs 80 feet in 5e), and you need a DC 39 Wisdom save to even successfully approach the thing (as opposed to DC 21 for 5e, which is an action rather than a passive).
3.5e tends to be considerably mechanically stronger than 5e, although by the numbers 5e creatures often have more hit points (albeit laughably less Armor Class- the difference here 39 AC, 627 hit points, and 20 Damage Reduction vs 19 AC, 256 hit points, and no damage reduction).