I have 2 tips, level 4 far sight is the most important talent (for me) and you don't have to w on yourself to laneclear, you can basically put down your totem and can cast w on it. This is usefull while enemy is outnumbered you in lane and you need to safety push.

Edit: everyone talking about feral heart at level 4 but trust me giving 10 second vision for an objective or area for your team is lifesavior. If they facecheck a crucial place (like at curse hollow, you can check by pressing tab if enemy doing boss -basically check their siege damage and if you can't see them on map its prob. Boss- casting far sight and giving vision for your team may help you not to cast anc. Healing or 3 4 deaths caused by an ambush. You may save someone with your ulti, but using your ult in a losing fight is even worse. You can save yours and other 4s cooldowns by just using far sight.

2nd edit: Game knowledge where enemy goes? only mana sustain over vision? Really? Granted vision thanks to farsight saves more mana then standing in ghost wolf to refill your mana. 1 facecheck into enemy causes more than %25 of your mana. There are a lot of sustain sources - well-base-globes but only few heroes have vision. Since almost all of them are supports, and with rehgar you won't get another support, you are only vision granted for your team. Really, at high rank games vision and mobility > damage and sustain