I think, when people are talking about 1e feel, they are talking about modules that evoke a fairly specific group of modules from the late 70's and early 80's, the list most likely containing the following:
A1,2,3,4 - Slavers series
B1 - In Search of the Unknown
B2 - Keep on the Borderland
B3 - Palace of the Silver Princess
B4 - Lost City
C1 - Hidden Shrine of Tomoachan
C2 - Ghost Tower of Inverness
D1-2,3 - Drow Series
G1-2-3 - Against the Giants
I1 - Dwellers of the Forbidden City
L1 - Secret of Bone Hill
N1 - Cult of the Reptile God
Q1 - Queen of the Demonweb Pits
S1 - Tomb of Horror
S2 - White Plume Mountain
S3 - Expedition to Barrier Peaks
S4/WG4 - Tsojcanth/Tharizdun combo
T1 - Village of Hommlet
X1 - Isle of Dread
X2 - Castle Amber
Superficially, these all have the same artistic deisign, format, and layout - i.e. brightly colored painted picture on front and back, slash on the upper left front, number-letter code, gatefold cover with mono-colored (either b/w or blue/w) maps on the inside, sparse art on the inside which was always black and white ink prints. An absolutely excellent group of artists worked on these - Otus, Rosloff, Dee, DSL, Willingham, some of Easley's early work, etc.
Outside the superficial art/design commonalities, these modules had the following:
1. They were short, generally 32 pages, but had a ton stuffed into them. Usually 9 or 10 point font, with 1/2" or less margins. Not a whole bunch of space used on monsters or NPC stat-blocks.
2. They weren't campaign setting specific. Sure, they generally told you where they could be located in Greyhawk or the Known World, but they were generic enough to be plopped down anywhere in a DM's campaign setting. They didn't assume that you had additional campaign oriented material (mainly because they hadn't printed any additional campaign oriented material for half this stuff).
3. Low plot content. Very non-linear. They essentially gave the DM a micro-setting, and left it up to the DM and players to determine why the PC's were there and what they were going to do.
4. Highly expandable. The DM could use them fine as written as a couple session adventure, but had enough guidance to base an entire campaign off of them.
R.A.