A few thoughts for you...
"It isn't complex enough."
D&D's combat system is indeed fairly abstract (though there's a lot of people out there who feel differently about the complexity thing, just so you know....they feel that 3E / 3.5 is *too* complex, and caters to rules lawyers and munchkins for that reason). If you and your group really want a more detailed combat system, D&D just may not be your cup of tea, and you may want to investigate other game systems.
That said, there are ways to make D&D combat at least not quite so "chessboardy." Read p. 50 of the DM's Guide, for thought about locations for combats. Having a combat in the rain, on a bridge, across a ledge, etc. are ways to make the combat feel more real and exciting, and less like it's taking place on a blank battlemat.
Also, you may want to look at the optional rules in Unearthed Arcana (particularly facing) that add a level of complexity.
"The enemies are a little too easy."
First of all, use the Encounter Level information in the DMG (p. 49) as a check to make sure you really are throwing appropriately difficult opponents at your PCs.
Secondly, the biggest thing I've always seen that makes enemies tougher are intelligence and tactics. My first DM (and one of my best friends) has this fatal flaw as a DM: he doesn't have a good tactical sense, so his monsters don't, either. They never surprise us, and they usually just stand there and swing at us. Planning and tactics on our part usually let us beat the tar out of his monsters without us having to break a sweat.
Granted, you shouldn't have oozes using sophisticated tactics, but intelligent monsters should use their intelligence. Let your monsters use flanking, trip attacks, etc. Do a Google search on "Tucker's Kobolds" (a classic example of wimpy monsters who could do great damage to a party, thanks to tactics).
A lot of "tough" monsters (from a CR basis) aren't just physically tough; they have a lot of abilities besides melee attacks that make them dangerous -- make sure you use those abilities to their fullest.
One of the toughest adversaries I ever sent up against my players was fairly simple: an archer with the Shot on the Run feat, and Dust of Disappearance. He was invisible, and nearly impossible to pin down (because he'd shoot as he was moving). Until the party's druid got lucky with a
faerie fire, negating the Dust, it really looked like this one guy might wipe out the party.
Finally, learn from your players. They'll undoubtedly come up with all sorts of funky tactics and ideas; steal these ideas liberally. Few things are better than using their own idea to club them upside the head.
