+1 to daupinous' comments.
There's nothing inherently wrong with what you're suggesting, but it does alter the basic underpinnings of how 3.5 works. You'll actually have an easier time of this at lower levels, where full casters (OK, wizards) are little more than commoners with a Good progression on their Will Save. Magic is less important here. Monsters tend to have fewer hit points, reasonable AC, and few (if any) special attacks. Once you edge past Level 6 or so, however, you're going to find that lack of full-on magical support will require you to be very careful in building your encounters. You'll need to give the PCs terrain they can exploit, consider expanding what they can accomplish with skill use, and give them a way out when things go bad.
You may want to remind your players that running is a viable tactical option. A lot of D&D gamers tend to think the DM would never intentionally kill a PC or put them into an encounter they cannot win through combat. However, there are very famous literary characters that actually spend more time running than they did fighting. Conan, for all his butt-kicking for goodness, what little more than a thief with a big-ass sword. He spent a lot of time running from sorcerers and superior force of arms.
dauph's example of DR X/Magic is a great example of what I'm talking about. With averaged damage around 7 or 8 (figure 1d8 damage die and a +4 damage modifier), DR 5 means each PC does only 2 or 3 points of damage per attack per round. Without terrain advantage, a single 2HD creature with DR 5/Magic suddenly becomes a serious threat in a toe-to-toe battle. Toss in some blocking terrain to give archer types s a few rounds of safe fire, stuff for the rogue to dart around to gain Sneak Attack, and a pit or a large rock and the encounter becomes an interesting game of cat-and-mouse.