One difference which I noticed last night, is that if you have PhyR / G.998.4 / G.INP, for ADSL2 there are differences in the standards compared with ADSL2+ (and VDSL2). See G.998.4 annex A.
For ADSL2, the maximum amount of RAM to be allocated for the ReTX Tx queue in the DSLAM, quoting - "transmit retransmission queue in the CO" - is limited to the half of the downstream interleaver delay in bytes, i.e.: Qtx* Q * H ≤ 8001 bytes". For ADSL2+ this figure can optionally be 12000 bytes instead - see annex B. The maximum value of Q*H is 1024 bytes, but I can’t see all the individual parameters for my modem.
So a constraint on performance is possibly relaxed with ADSL2+, by an implementation option. This will affect the range of patterns of lookback retransmission that can be supported, where a receiver can indicate a NACKed or ACKed DTU way back which is different from its successors. When such a circumstance could practically happen is quite unclear to me, and even the mere possibility is an option.
A more careful look might reveal other differences between ADSL2 and ADSL2+, but I have yet to do all the tedious work.