Is it possible to tackle hero quests or any of the other harder multiplayer quests alone if you level up your character enough? Or is joining up with other players the only way to get through those missions? I barely have time to game these days but really enjoyed KOTOR and have been thinking about getting TOR. Can I play through each character class's main story alone, or will I hit a wall where multiplayer is absolutely required and won't be able to progress. The sub fee is sort of holding me back, but I've heard really good things about the story.

You can solo the entire main story line just fine. The only time grouping is needed is when you do the Heroics/Flashpoints, and those are not needed to complete the main story.

Don't overlevel and go back and do heroics unless you hate yourself. You won't get anything out of it, and the actual writing is about as hit-or-miss as any of the other side quests. There are more than enough single player quests to get you to 50.

When I was playing, it wasn't hard to get groups for heroics, and it's a fun game to play with other people. Join a Giant Bomb guild.

You can do it all yourself if you level enough. It tends to be very boring to grind that out though. If you're still feeling compelled to play, I'd suggest you just stick to the main storyline and normal side quests. That will keep you leveled enough to keep moving. Engaging in more than that solo will crush your desire to keep playing. I know it did for me.

If you over level for the multiplayer quests your not going to get anything useful out of them. Unless your severely under-geared.