I use my MTB for commuting mostly (150km/week) with a bit of non-challenging trail riding with my 10yo thrown in.
What I've found over the last 6 months might be useful to you.
1) Don't skimp on hardware.
I've had no end of trouble with my drive train. The problems were initiated by the front derailleur not being installed properly. This allowed the chain to jump off the middle chainring, miss the small one entirely and jam itself around the bottom bracket when changing down. I thought I had it nailed after changing the front derailleur to a Shimano Hone a few months ago, but last week it started doing it again consistently, so now I've changed the middle chainring to something decent so hopefully the teeth won't bend over again!
I'd say Shimano Deore would be your minimum entry level if you rack up the k's like I do and you weigh a bit (I'm 80kg)
2) Get some decent road slicks for commuting
It's safer. Knobblies have a habit of breaking loose suddenly on the tarmac, and decent slicks are far, far easier to pedal anyway. Continental Sport Contact 1.3's are the go, and have puncture protection built in.
Keep your knobblies that came with the bike. Changing to them for weekend trail bashing can be accomplished in about 15 minutes - if you're slow!
A decent floor pump wth pressure gauge is a big help.
3) Get some lockout forks
You want your sweat to push you forward when climbing. Bob bob bobbin' along just heats your ... shock oil.
4) Bike shop mechanics are useless.
Go to Torpedo7.com.au and buy yourself the $59 toolkit. Then buy yourself a copy of Zinn and the Art of Mountain Bike Maintenance and invest some time in a little education.
5) Clipless pedals are definitely worth the investment
But they do take some getting used to. I've had the usual couple of really stupid falls off my bike 'cause I couldn't get my feet out when I came to a sudden stop, but they've saved me a couple of far worse accidents at speed on bumpy terrain when I would definitely have been tossed off the bike. And they're vital in wet weather like this morning.