TL;DR: The Raspberry Pi 3 Model B+ is a nice update that provides, better networking with WiFi now 802.11.b/g/n/ac, Bluetooth 4.2/BLE and Gigabit Ethernet (although limited to 300Mbps). For Makers, the Raspberry Pi Foundation has made it easier to incorporate the RPi3B+ into end products keeping the RPi3B+ in production until at least January 2023 and providing "modular compliance certification" for the wireless module.

Source: https://raspberry.piaustralia.com.au/blog/new-raspberry-pi-3...

WHAT IS THE SAME:
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- Same mechanical footprint as both the Raspberry Pi 2 Model B and the Raspberry Pi 3 Model B.
- Same GPIO pinout, so no need to change your hats/shields/daughter boards
- Same power supply recommended (5V/2.5A DC via micro USB connector)
- Broadcom BCM2837B0, Cortex-A53 64-bit SoC (System on a Chip), but this time clocked to 1.4GHz, which yields roughly a 10% performance increase over the Raspberry Pi 3 Model B. The chip now has a metallic heatsink and bears the markings BCM2837B0 1FSBG, TA1738 P20 W39-01 N2 W.

WHAT IS NEW:
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WIRELESS MODULE:

The product briefing document from the foundation states that the onboard wireless module now supports dual-band 2.4GHz/5GHz IEEE 802.11ac and both Bluetooth 4.2 and BLE (Bluetooth Low Energy).
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The wireless module in the RPi3B+ is not mentioned in the product briefing document and is hidden under shielding. Prying open the shielding revealed a Cyprus Wireless CYW43455 (datasheet).

- The CYW43455 supports the following WiFI standards: 802.11ac/n/a/b/g/d/h/i
- Security: (WEP/ WPA Personal / WPA2 Personal / WMM /WMM-PS (U-APSD) /WMM-SA / AES (hardware accelerator) /TKIP (hardware accelerator) / CKIP (software support)
- Proprietary protocols (CCXv2, CCXv3, CCXv4, CCXv5, WFAEC
- IEEE 802.15.2 Coexistence Compliance (on-silicon solution compliant with IEEE 3-wire requirements)
- The CYW43455 supports the following future IEEE drafts/standards: 802.11r (fast roaming between APs), 802.11w (secure management frames), 802.11 Extensions:, 802.11e QoS Enhancements (as per the WMM specification is already supported), 802.11h 5 GHz Extensions, 802.11i MAC Enhancements, 802.11k Radio Resource Measurement

The CYW43455 data sheet states that the Bluetooth core supports:

- Bluetooth 2.1 + EDR
- Bluetooth 3.0
- Bluetooth 4.2 (Bluetooth Low Energy)

The Wireless LAN module now has "modular compliance certification", which allows the board to be designed into end products with significantly reduced wireless LAN compliance testing, improving both cost and time to market.

NEW GIGABIT ETHERNET:

The new RPi3B+ has support for Gigabit Ethernet using Microchip's LAN7515 (which is a USB 2.0 to 10/100/1000 Ethernet Bridge with 4-port USB 2.0 Hub). The LAN7515 supports a max number of 6 downstream USB ports and 1 Ethernet Port limited to a maximum throughput of 300 Mbps.
The LAN7515's ethernet controller supports numerous power management wakeup features, including Magic Packet™, Wake-on LAN (WOL) and Link Status Change.

POWER:

The RPi3B+ is Power over Ethernet (PoE)–enabled, but requires the purchasing of a separate PoE HAT.

PRODUCTION LIFETIME:

If you're thinking of incorporating the Raspberry Pi 3 Model B+ into another product, the Raspberry Pi foundation has committed to keeping the RPi3B+ in production until at least January 2023.

HEAT MANAGEMENT:

Due to the BCM2837 now running at 1.4GHz, the RPi3B+, runs relatively warm. The foundation provides the following warning: "This product should be operated in a well-ventilated environment and, if used inside a case, the case should not be covered". Given the optional "open lid configuration" of the official Raspberry Pi case I asked for clarification, and their Director of Product Management came back with: "It only means if you have it in a case the case itself should not then be covered".