very often, they cant block at all and watch your creatures run rampant over their planeswalkers and life totals
Mana Web fits into (at least) that artifact with unique effect category, as it forces opponents to tap ALL lands they control that could produce any type of mana that land could produce upon them tapping a single land for mana

Dolliv - 022/198 #22Common $0.13 Breloom #4Uncommon $0.11 Smoliv - 020/198 #20Common $0.17 Meowscarada - 015/198 #15Rare $0.24 Quaquaval #54Rare $0.21 Houndour - 033/198 #33Common $0.07 Skiddo #11Common $0.11 Houndoom - 034/198 #34Common $0.23 Tarountula - 017/198 #17Common $0.14 Fuecoco #36Common $0.16 Drowzee #49Common $0.89 Drowzee #49Common $3.12 Drowzee #73Common $0.57 Drowzee #54Common $0.61 Drowzee #73Common $0.73 Drowzee #54Common $1.04 Drowzee #67Common $4.68 Drowzee #65Common $0.42 Drowzee #62Common $0.52 Drowzee #35Common $0.25 Pokemon 2-Player Starter Set (Revised Base Set Reprint Run) $249.50 Base Set Theme Deck - "Blackout" (Revised Base Set Reprint Run) Base Set Theme Deck - "Zap!" (Revised Base Set Reprint Run) $499.95 Base Set Theme Deck - "Brushfire" (Revised Base Set Reprint Run) Base Set Theme Deck - "Overgrowth" (Revised Base Set Reprint Run) $685.00 Base Set Booster Pack [Revised Unlimited Edition] $786.87 Base Set Booster Box [Revised Unlimited Edition] Base Set 2 - 2-Player CD-ROM Starter Set $176.60

On a well-supported type like vampire, convoke feels like a cost reduction since good vampires can come down before this and gladly for it
With that said, not all of the original card designs will be making their return, but there are still plenty to look forward to
Abzan and Jeskai rose to the top during Khans of Tarkir Standard, and its likely a couple guilds will rise to the top of Guilds of Ravnica Standard as well, but Im not just going to fill my list with all the cards from a couple guilds