A map pack is much smaller in content than a full game. The content in Factions and Nightfall is plenty to make them full games. That they use the same mechanisms as Prophecies is not an issue, lots of games are essentially little more than more content but with the same skeleton as their predecessor. By most common definitions, GW1 is 3 games and 1 expansion, despite the common core. Though for the purpose of judging innovation, these 4 titles have too much in common to be considered as showing a trend.
The philosophy of ANet can be seen in three ways: (1) innovation, (2) fixing existing common problems with the genre, (3) low grind. If not for those three aspects, ANet making a MMORPG with persistent zones would give us another WoW clone in the same way we got Rift and SWTOR. But that's not what we got. I'm not going to waste time giving data to back that up, the data is obvious to anyone but the mentally blind (e.g. those who care to win an argument more than finding the truth, e.g. jam).
Consistency can also be across features. For example, ANet has consistently been addressing existing common problems with the genre, and this can be seen with resource nodes that many can sample, dynamic events that scale, the removal of kill stealing (or most of it anyway), the scaling of players, the flattening of the levelling curve, etc.