Okay I started with RQ2 and ran RQ3 for several years, so I guess I qualify, but the short of my reasons i love MRQ:
1. It's streamlined; all the snaggy bits from prior editions are missing (skill application in char gen, CAs instread of strike rank actions, fatigue and encumbrance rules that work, etc.)
2. Non-Vancian Magic: The magic system continues to permutate in to something distinct and flavorful from TOG.
3. "Reality" Based: Barring the slight chance of legendary abilities becoming a reality in any games I run (and I like 'em anyway), MRQ maintains a more visceral and reality-grounded feel than other games, and maintains the same feeling from older editions of MRQ.
4. Non-Glorantha Support: Loads and loads of support. Not just Glorantha support (I do not do Glorantha, at all), but the kind I can actually use for my own campaign. Amazing!
5. Slicker and Faster Combat: Combat is cleaner and smoother than in RQ3. Yes, that's right, I think the MRQ system is much more efficient than the older editions. My acquisition of the BRP proof reminded me just how muddled and tedious the minutiae of old RQ3 combat could get; luckily the vast majority of the combat rules in BRP are presented as optional; but I was surprised at how when I started working on my BRP campaign I found myself wanting to borrow from MRQ to modify BRP and not the other way around......!
6. Good Deal: MRQ is essentially the best buy on the market with Runequest Deluxe. I don't own a laptop and dislike PDFs, so I find games which are easily ported to be my favorites these days. That makes it Runequest Deluxe, BRP, and Castles & Crusades so far. Most other games come with lots of baggage, unfortunately (sorry, GURPS....D&D....anything White Wolf....too much!)
7. Lots of Support and Recognizes it's Literary Roots: The Spellbook is absolutely awesome. I can't wait for the Necromancer's Book. This kind of support was sorely lacking from prior editions of the game. Having Lankhmar, Pirates, and all these Eternal Champions and historical books here and on the way is very nice. In a time when TOG is trying to capture the video game/MMO player's attention, MRQ is still recognizing the literary roots of fantasy.
8. The SRD. Nuff said!