I haven't been in the beta, but Spinks made a round-up and it sounds like the expansion was in pretty good shape other than the mounted combat system. It's certainly possible that this is part of the problem - mounted combat is reportedly a substantial chunk of the expansion, and some of Blizzard's experiences with vehicle/dragon combat in the Wrath era suggests that players do not love being forced into something that changes their playstyle. Some of LOTRO's game systems already have a bad reputation for being grindy, and I could see some concern if this one was also buggy to boot.
That said, the more I think about it, the more I'm wondering that the real problem is the release date calendar. Turbine claimed the September 5th date back on June 5th, but on June 28th Guild Wars 2 declared that it would "launch" (early access aside) a mere week earlier, on August 28th. Blizzard responded by placing WoW's expansion on September 25th, and later added to the scuffle by putting patch 5.0 with talent changes and other system updates on August 28th.
Turbine isn't new to this game, and they're not afraid to launch against competition - the Moria expansion launched the week after Wrath of the Lich King. (Then again, Turbine has never placed another product in physical stores since that day.) That said, I'm wondering if they just didn't like their odds of seeing their expansion drowned out by most of the known MMO world descending on either WoW or GW2. They'll still be up against Rift with its un-announced expansion launch and possibly some other tidbits (perhaps the end of the year EQ2 expansion, or SWTOR's relaunch), but I like their odds better in October than right here and now.