My personal experience in the world of Telara was that it was technically well executed but very dry solo. Where the game shined was in groups, and I spent more time leveling with other people than I have in any other MMO before or since. When I read over accounts from people who have stuck with the game, it seems that most have done so because their friends or their guilds have chosen Rift precisely because it is at their best when enjoyed in good company.
Now, in Guild Wars 2, you have a game that was supposedly developed under a philosophy where the first question was always how systems would affect players' ability to cooperate. Servers and levels, probably the biggest barriers between players, are functionally gone. The subscription fee is gone, and with it the constant financial incentive to quit the game. Tapping mechanics that cause other players in the area to become competitors rather than collaborators are gone. Open world events that encourage cooperation are in. Ironically, Blizzard dodges a bullet by having a major competitor choose not to tackle WoW head-on, while Trion's model is most similar and most in the crosshairs.
The Trion Response
Wilhelm says that his Rift server got pretty deserted during the GW2 prelaunch events, which is certainly anecdotal and probably a common experience around the MMO world this week. However, the approach that the GW2 devs had planned was no surprise given the game's lengthy and relatively public development cycle. Thus, Trion has had time to respond.
Within a few months of GW2's anticipated release, Rift has added a popular auto-mentoring system (much like GW2's approach), removal of faction barriers to grouping (which GW2 intentionally never had), and a new world PVP system (albeit with some kinks) to join longstanding features like free instant server transfers and cross-server groups. And yes, incidentally, a discount on a year-long subscription paired with an expansion - a move that Blizzard tried at the SWTOR launch, when it was widely regarded as a transparent attempt to lock in revenue before players canceled their subscriptions to go play a new game. The fact that the competitor lacks a monthly fee can't help that math any.
I don't expect Trion to fold anytime soon. Even so, they may have as much riding on where the dust settles after this launch and their forthcoming expansion as any other game on the market.