Although I prefer B/X and OD&D for rules, I usually do not care for the sorts of adventures that have players meandering in a dungeon for session after session. At least not in a continual fashion. A good example of this is Return to the Temple of Elemental Evil; which was fantastic until you actually got to the temple and are besieged with battle after battle, room after room, in an endless extermination procedures. I suppose this is because role-playing takes a backseat to fiddly mechanics and it is very repetitive.
My favorite types of adventures are exploration ones, where there is in fact little plot to speak of. Really these are just strange places for the PCs to meander around finding stranger things in a wide-open fashion. Isle of Dread and Dwellers of the Forbidden City are two such adventures that are almost completely non-linear with no beginning or end. I think every time I have ran these, the end result is usually very different depending on the group. Most importantly this can be a very good device where the PCs tell you what they want (even if they don't know it); mystery, action, investigation, heavy RPing, starting kingdoms, whatever. From there you should have a very good idea of what to run next.
Anyhow, I have run enough separate campaigns to put forth a little methodology that I construct for the campaign. Really I think the actual rules are fairly moot, so long as they don't bog things down overly (ala 4th edition, heh). B/X, OD&D, 1st-2nd, 3.5 edition, WFRP and so forth are all perfectly fine so long as the players are comfortable with it.
First Adventure - Usually a one shot homebrew adventure that is heavy on starting RPing in the start with a small excursion of some kind for action and accomplishment. I will be posting a typical starter adventure in my next post (Barnacle Men). This adventure is purely for the PCs to get to know one another and find their 'voice'. Also to get to know their class. I usually build the encounters to specifically cater to the played classes so everyone is doing something. Players can write a background if they want, but I prefer if they wait until after this adventure to do it since the players perception often changes after this first game.
Second Adventure - Usually multi-session. I often use a published adventure here since they are (usually) well balanced. For an urban area I like Assassins Knot or The Veiled Society. For non-urban areas, something like Night's Dark Terror work very well. I also like The Sentinel/The Gauntlet, the Saltmarsh series. These are not overly difficult and let the players act like a team as well as stand out individually. They also have a good climax and sense of accomplishment to end it up with. I usually make enough modifications to keep it fresh, and so that only a few monsters are recognizable (IE, no 'you see 10 goblins', instead PCs should be saying ' what the hell are those).
Third Adventure - Here is where I insert the exploration adventure. It can be as simple as finding a map or a contact informing them of a strange place. The PCs at this point should have enough resources to travel around. As mentioned before, Isle of Dread and Dwellers of the Forbidden City go well here - although I tone them down for lower level PCs. The lost Valley in Night's Dark Terror also works well. I have also homebrewed a few of these since they are great fun to write. This exploration adventure I feel is the most important adventure you will probably run for a campaign since they PCs can do whatever they want, and they likely have the gold to do many different things. It will probably set the tone for the rest of the campaign. Most importantly it tells you what the PCs want in their game so it is a good idea to set up several available options. Rumors of lost artifacts, evil cults sacrificing nubile maids, strange boats the sail up waterfalls, weird bark-men who ritually shave the native animals, bizarre murders among the native people, two warring factions with political intrigue - whatever. Give the plenty of options and see what they come up with. Alter progressive sessions accordingly until it runs it course.
Fourth Adventure+ - From here on out it should be rather distinctive on what the PCs want as far as a game. I generally set up a large campaign-wide plot at this point and have any adventure here on out based upon it, even if its minor. It can be something grand like thwarting the World-Emperor, building a castle and maintain a realm, or smaller things like giving an old man an honorable death or avoiding a particularly nasty ex-wife of a PC (yes, both have happened). The actual adventures I run at this point are usually a mix from a variety of adventure types and sources. I will do a homebrew followed by a published adventure followed by a purely out-of-my-ass spontaneous thing followed by a set-piece battle with a 100 minis on the table and so forth. Variety is important, so long as it is somewhat linked together as strongly as the PCs would like and that they follow the ultimate enjoyment that the group is looking for.
I have no aversion to new adventures and many good ones are around. In particular I like a lot of the old short-but-sweet adventures from Dungeon magazine. They often have a lot of flavor with out repetitious game play. I also freely plunder adventures from other game systems, new and old. I rarely modify them before hand, and instead just simply do that sort of thing on the fly as I run combat.
And so, there you go. Most of my campaigns will typically run between 6-18 months. I think after 18 months the players get bored with their PC or the rules or the setting. This is not a bad thing by any means and I think it is just as important to recognize when to end it, and end it on a good note. Ending it with a major story arc, a massive battle with siege engines and castles, going out in a blaze of glory and so forth.
Valley of Blue Snails was actually going to just be one of these 'exploration adventures' and nothing more. It was only later that I decided to make it a setting since the ideas were getting too large for a single adventure. I will still include an actual Valley of Blue Snails location and likely make it an open-ended exploration adventure. In fact I would like the Valley to be an all-inclusive area, sort of a newbie land for levels 1-5 and cover the first 3 adventure types I had listed above. As a side note I plan on placing the actual valley west of Kabdoria, near the river and between the many pink blotches of demi-humans.