It does get boring, because as I said I use one skill almost exclusively, plus fireball for single target. I'd love to experiment with other skills and gem synergies, but short of checking the wiki to see whats available and spamming trade chat to buy it there are no other options atm. Plus the more I'd specialize the build for this playstyle with passives (say more cold damage + chill) the more I'd be shoehorned into using it, so I've been avoiding them for now and just taking general caster traits. Titan Quest with respec mod was really more fun in that regard.
The end game is maps. And maps are a lot of fun. So you are better off going through all the levels.
Once you start finding good 4 socket items then you can diversify to a 7 or 8 different skills. You will get more gems from quests and you will start finding support gems and higher level skill gems.
The end game gets really complex when you have 4 gem sockets on boots, gloves, helmet, 5 or 6 sockets on your armor, and either 5 or 6 sockets on a two handed weapon, or 4 on two different 1-handed weapons or 4 on a one handed weapon and 3 on a shield. You can send up with over 20 sockets and at that point you get to play around with a whole lot of skills and options.
Starting over will just mean more of spamming of one skill. You don't really get to the more complex parts of the game until 4, 5 and 6 socket items start dropping.
With my Templar I will be using Glacial Hammer, Ground Slam, Molten Shell, Shield Charge, Heavy Strike, Tempest Shield, a few auras and maybe Leap Slam, and also a curse. And there will be at least a dozen support gems I will be using, plus some skills and support gems that I will only use for specific maps. And I will also need to use a totem in certain situations.
Maps sometimes change the rules of the game. So for instance you might find a map where all monsters have fire resistance, or where all players have blood magic, etc... you have to change your set up to do some of the these maps.
Also had my first HC death in Open Beta yesterday - only a level 23 Shadow, but it still stings since I had equipped him with some pretty sweet rares and it was due to my own carelesness. I was in Chamber of Sins 3 and ran in too eagerly to kill a Sin Lord, hoping to get him down before he resurrects too many casters - I didn't, he did, and the casters gibbed me.
I usually carry around a Detonate Dead skill gem that I pop in when I have to deal with necromancers. In maps and in higher level play necromancers can be a real problem, but Detonate Dead can control them... so I recommend carrying one around that you can swap into your set up. It doesn't even need to be leveled up.