What in particular are you attempting to restore? Just having a folder ("Macintosh HD") open won't work (and never has, that I know of). You always need to select a file or folder.
For what it's worth, I just entered TM myself, using a single backup that dates back to 2011, and I'm able to select any date/time, click on an item, and then can use either the Restore button or the action in the tool bar (they're enabled). It seems noticeably more responsive than Mavericks (I hadn't really entered TM until tonight). The drive is in a FW800 MacAlly case, though that shouldn't make a difference.
If you are trying to restore a complete volume from TM, the only way to do that is by booting into Recovery HD, or starting the Installer (which will restart), or booting to a separate bootable volume of the Yosemite installer (like a USB drive).
(Currently running the public version of 10.10.1.)
Edit: well, even though I had no problem using Macintosh HD, I'm finding some oddities when starting from, say, "Documents". It starts out looking like there are no older backuos, but if I navigate up to Macintosh HD, then back down to my Documents folder, the past becomes clear, so to speak. Then, I tried going over to Desktop via sidebar, which looked ok, then back to Documents, I again lost the past. Going back up the hierarchy, then back to Documents, and I was able to regain the past. So, yea, something not quite right, but far from non-functional, especially with no starting folder upon entry to TM (it defaults to Desktop, and works properly). For the most part, TM worked fine.
Some of what I'm seeing could be due to such a large (time-wise) TM backup, which contains at least a few full backups which used to be triggered upon changing machines or drives. Between 2011 and 2013, I had changed internal drives a couple of times (more space, then SSD), before changing machines to the rMBP a I have now.
At least the fan-revving star-field has retired. While cool looking, and serving its purpose in separating user environments, I always felt it was such a waste of power. It also made remote troubleshooting by screen sharing a less than "stellar" experience.