If you're a min/maxer, then you'll definitely want to invest a lot of points into blacksmithing and sagecraft. It's not really necessary for the game though, since the loot you find is generally good enough to get the job done with ease.
If I had to pick one, I'd go with sagecrafting to tack on a few gems to your weapon/armor slots. Blacksmithing does get you better gear than what you'd find, but like I said, it's hardly necessary.
Personally, I say focus on Detect Hidden first and foremost, then maybe Persuasion to get more out of the story/dialogue. If I had a lot of skill points to invest, I'd probably up dispelling too, since those minigames are such a pain.
Other than Detect Hidden, I've been splitting my SP between Blacksmithing and Sagecrafting, and the combination of the two has made the non-combat part of the game infinitely more fun for me. I love to take a break from questing and fighting to forge better gems, salvage my drops, and then smith some bad-ass gear. The Smithing is terrific because it really enables the player to min/max around their preferred character type; and it only becomes more and more useful as the game progresses.
I'm a level 17 Finesse/Might mix, playing on Hard, and am about 25 hours into the game; and so far, other than recipes and Minor Healing Potions early on, I've only purchased maybe half a dozen gear-related items from the vendors. Between the drops and my leveled-up Sagecrafting + Smithing, money has become rather useless at this point.
I love knowing that, at any time, and can pay a nominal fee to completely respec. my character--this takes a way a ton of the pressure surrounding the concern of messing up a build. Oh, and it lets you delve into the huge variety of combat options w/o starting a whole new game.
Loving this game even more than I anticipated, excited to see what the future holds for 38 Studios.
Also: I would love a patch that allows the UI gauges to fade out when not in use b/c I'm tired of the Image Retention they cause on my Panny Plasma. Maybe it wouldn't happen if my gaming sessions weren't 3-4 hours at a time, but, hey, it's their fault for making such an awesome game.