1: Most HO scale plastics are not UV compatable. Peco track is, but locos and rolling stock, structures etc are not. It is important and makes a difference. UV light will rip plastic to shreds in a short time without protection.
2: Physics. Every little leaf or gust of wind will blow your train off the track. You would have to weight each peice of rolling stock quite a bit, so expect to use M.U. or fewer cars in a train. Prepare for frequent derailments.
3: Rain, birds, animals, leaves, bugs, and other 1:1 scale real world issues. Even in G scale these can cause problems, multiply by a factor of 500 on the irritating scale for HO. HO loco motors and drives are not weather sealed and every peice of dirt will find it's way into the gearing.
4: All your electrical items would have to be easily detachable and brought inside when not in use. HO power packs and switch machines are simply not designed for outdoor use.
5: Prepare for some seriously filthy track at the begining of each operating session. Slugs leave a film, and dirt loves train tracks.
Now, with all that said, there are several very succesful HO layouts in the world today and I would never try to deter anybody from experimenting and learning. I would recommend not doing this as the technical problems are not easily overcome and it could easily overwhelm a beginer's knowledge and patience.
You might be surprized at how much G scale (with 4ft radius curves) can fit into a corner or around a tree or bush! Why not buy a Bachmann Big Hauler set at an after Christmass discount sale (junk the B'mann track and buy Aristo-craft for outdoor use) and give G scale a try?
