well, some proprietary parts, depending on model are.... floppy drives, power supplies and memory.
The floppy drives on some of the Dell boxes are just bare drives with no front faces so they can fit in the pre-molded dell case. But just try and find a floppy drive that will fit those bays without buying it from Dell. If you find one let me know because I'm tired of charging people an arm and a leg for that replacement.
Power supplies are another issue. I'm pretty sure that the PS in recent models are shelf replaceable, but after crap that Dell has pulled in the past, I wont pull off the shelf to replace customers' parts. More info on this page,
http://www.endpcnoise.com/cgi-bin/e/dellconverter.html but main quote off of it follows
Memory is the same story in all PC manufacturer's boxes, just go to Crucial or Kingston's web site configurators and see why all pc2100 memory is not available for some ddr dells( or others). We stock lots of memory in the store, both Crucial and Kingston. but if a customer comes in for an upgrade, I'm sure as hell gonna check with those configurators before I pull something off the shelf. Last week a customer had bought a stick of PC2700 from another store and it wouldn't work in his Dell. I checked the specs and Crucial said that that specific model of Crucial memory was NOT COMPATIBLE with that specific DELL model. But, two of their other lines of PC2700 were compatible. Of course this can happen for almost every commercial PC out there.
Like I said, just trying to relate some of the experiences I've had with Dell in the last 18 months, not trying to tick anyone off.
I don't know what percentage of market share Dell has on the computer market. I'll take a wild assumption and say that 50% of all computers sold are Dell. And that is a very broad assumtion. I dont think they have anywhere near that many sales. Now, add the FACT that as a computer REPAIR shop, 70% of the computers that have walked through the door in the last 2 years are Dells, I wouldnt steer anyone towards them. Ever. End of Story.