All systems tend toward centralization whether planed or unplanned. DPOS gives the end use as much control over that centralization as possible where as the other systems the small player has less control.
1) All systems are subject to the 51% attack, it is inherent in the nature of consensus itself.
2) Economics limits the number of nodes that can profitably validate for a given transaction fee.
3) Peercoin / Nxt centralizes around the largest account holders who are the only ones who can profitably validate at competitive fees.
4) Nxt supports a form of delegation (leasing)
5) Bitcoin uses Delegated Proof of Work (mining pools)
Primary weakness of DPOS is voter apathy or lack of sufficient knowledge. A divided user base is less secure than a united user base. These issues are being mitigated through user interface enhancements and the latest refinement to DPOS (RDPOS, Recommend DPOS) whereby users trust others to make recommendations on whom they vote for.
Advantages on DPOS vs PPC and NXT:
1) Faster, Scales Better, Confirms Quicker
2) Accounts & TITAN make it easier to use + more private.