What are the differences between NR PDCP & LTE PDCP?

5G-NR PDCP  is derived from 4G-LTE PDCP but with some additional responsibilities.

In my view there are majorly two points you should remember.

Point 1: NR-PDCP has a very important functionality of supporting split bearers & Dual connectivity. So, in this case packets will be reaching from 2 nodes (4G+5G or 5G+5G) at NR-PDCP. So, it has to do the Routing, Duplication, Duplication detection, Reordering etc

Point 2: In 5G we can do Integrity protection & Integrity check for user plane data too along with control plane data.

The highlighted parts in Red & Green are updated in 5G & 4G.  Let's first see the diagram of both NR-PDCP & LTE-PDCP.

Click on the image to enlarge

Let's start from transmitting side first:

In 4G PDCP, Integrity protection is applicable only for Control plane data. 

Whereas, in NR-PDCP, Integrity protection is done for both User and control plane data.

In 4G PDCP, for split & LWA bearers, Routing of  user plane data is only applicable.

Whereas, in NR-PDCP, Routing of  user plane data & control plane data is applicable for split bearers.

In 4G PDCP, Uplink Data compression and decompression is applicable.

Whereas, in NR-PDCP Uplink Data compression and decompression is not applicable.

At reception side:

In 4G PDCP, Integrity verification is applicable only for Control plane data. 

Whereas, in NR-PDCP, Integrity verification is done for both User and control plane data.

In 4G PDCP, for split & LWA bearers, Reordering of  user plane data is only applicable.

Whereas, in NR-PDCP, Reordering of  user plane data & control plane data is applicable for split bearers.

In 4G PDCP,  In-order delivery and duplicate detection of the packets are applicable only for User plane data.

Whereas, in NR-PDCP, it is applicable for all the packets based on configuration.

NR-PDCP can do duplicate detection and discard of duplicate SDUs but cannot be done by 4G-PDCP.

There are some other differences like:

4G-PDCP supports 5, 7, 12, 15, 16, or 18 bits PDCP-SN but NR PDCP supports only 12 & 18 bits PDCP-SN.

4G-PDCP has supports extra PDU type such as LWA status report, LWA end-marker packet & UDC feedback packet.

4G-PDCP supports some extra SDU type such as ARP & PC5 signalling.

These are the brief differences between NR-PDCP and LTE-PDCP.

Reference: 

3GPP Spec 36.323 v16.02 for LTE-PDCP 

3GPP Spec 36.232 v16.06 for NR-PDCP