I'm not the biggest fan of classic D&D, but one thing it does have is great lists: spells, magic items, monsters. Generally I like the AD&D ones better than the 3E ones. I converted a lot of them for my old Rolemaster campaign.
I find the overall vibe of Runequest very appealing - especially the Gloranthan take on magic, and the different forms of magic - even though I find the mechanics too spartan for my actual play preferences.
Classic Traveller has great PC generation. I don't particularly want to play those PCs, but they are a lot of fun to create.
4Ed: level-based benefits spread out more evenly; 3rd level spells gained at 3rd level, 6th at 6th, etc.- most intuitive design in D&D's history; overall improvement of the Warlock; Rituals are at least a good idea.
GURPS: some of the best researched & written sourcebooks out there.
RIFTS & Space: 1889: hate the systems, but the fluff is awesome!
Speaking for myself, I didn't do it as a sneaky putdown- I'm just trying to be clear.
For example, despite my dislike of the RIFTS system, I probably have a solid 5'+ of shelf space devoted to it. Only my favorite games- HERO and 3.5Ed- take up more. And its purely on the strength of the fluff. Likewise, I have at least 4 copies of the core Space: 1889 book, plus a nearly complete collection of the rest of it. But the system isn't all that great.
If I spoke to my wife like that I'd get a kick in the junk
I like the layout and clarity of 4e's presentation as a rulebook to reference.
I like the clarity of 4e's action sequence.
I thought the idea of picking offensive and defensive stats is a good start to build on even if I think 4e's actual implementation is clunky and, I think, actually detracts from 4e's balance.
I like some of the idea of once/encounter combat moved for martial characters.
I like the idea of rituals that exist with a separate resource pool from combat-oriented abilities.